Fix malformed sig parsed as multi-target

[thomasmarshall]

This fixes a crash when Prism parses a malformed sig as a multi target node:

sig,{params(x: Integer).void}

This is just an example that came out of the fuzzer, it's not specific to sigs, it's how Prism parses the comma in the malformed code:

@ MultiTargetNode (location: (1,0)-(1,29))
├── flags: newline
├── lefts: (length: 2)
│   ├── @ LocalVariableTargetNode (location: (1,0)-(1,3))
│   │   ├── flags: ∅
│   │   ├── name: :sig
│   │   └── depth: 0
│   └── @ HashNode (location: (1,4)-(1,29))
│       ├── flags: ∅
│       ├── opening_loc: (1,4)-(1,5) = "{"
│       ├── elements: (length: 1)
│       │   └── @ AssocNode (location: (1,5)-(1,28))
│       │       ├── flags: ∅
│       │       ├── key:
│       │       │   @ CallNode (location: (1,5)-(1,28))
│       │       │   ├── flags: ∅
│       │       │   ├── receiver:
│       │       │   │   @ CallNode (location: (1,5)-(1,23))
│       │       │   │   ├── flags: ignore_visibility
│       │       │   │   ├── receiver: ∅
│       │       │   │   ├── call_operator_loc: ∅
│       │       │   │   ├── name: :params
│       │       │   │   ├── message_loc: (1,5)-(1,11) = "params"
│       │       │   │   ├── opening_loc: (1,11)-(1,12) = "("
│       │       │   │   ├── arguments:
│       │       │   │   │   @ ArgumentsNode (location: (1,12)-(1,22))
│       │       │   │   │   ├── flags: contains_keywords
│       │       │   │   │   └── arguments: (length: 1)
│       │       │   │   │       └── @ KeywordHashNode (location: (1,12)-(1,22))
│       │       │   │   │           ├── flags: symbol_keys
│       │       │   │   │           └── elements: (length: 1)
│       │       │   │   │               └── @ AssocNode (location: (1,12)-(1,22))
│       │       │   │   │                   ├── flags: ∅
│       │       │   │   │                   ├── key:
│       │       │   │   │                   │   @ SymbolNode (location: (1,12)-(1,14))
│       │       │   │   │                   │   ├── flags: static_literal, forced_us_ascii_encoding
│       │       │   │   │                   │   ├── opening_loc: ∅
│       │       │   │   │                   │   ├── value_loc: (1,12)-(1,13) = "x"
│       │       │   │   │                   │   ├── closing_loc: (1,13)-(1,14) = ":"
│       │       │   │   │                   │   └── unescaped: "x"
│       │       │   │   │                   ├── value:
│       │       │   │   │                   │   @ ConstantReadNode (location: (1,15)-(1,22))
│       │       │   │   │                   │   ├── flags: ∅
│       │       │   │   │                   │   └── name: :Integer
│       │       │   │   │                   └── operator_loc: ∅
│       │       │   │   ├── closing_loc: (1,22)-(1,23) = ")"
│       │       │   │   └── block: ∅
│       │       │   ├── call_operator_loc: (1,23)-(1,24) = "."
│       │       │   ├── name: :void
│       │       │   ├── message_loc: (1,24)-(1,28) = "void"
│       │       │   ├── opening_loc: ∅
│       │       │   ├── arguments: ∅
│       │       │   ├── closing_loc: ∅
│       │       │   └── block: ∅
│       │       ├── value:
│       │       │   @ MissingNode (location: (1,28)-(1,28))
│       │       │   └── flags: ∅
│       │       └── operator_loc: (1,28)-(1,28) = ""
│       └── closing_loc: (1,28)-(1,29) = "}"
├── rest: ∅
├── rights: (length: 0)
├── lparen_loc: ∅
└── rparen_loc: ∅

Here, one of the lefts is a HashNode (the {params…} part), which is obviously not valid.

The fix I came up with, is to check each of the lefts and rights to ensure they are valid nodes, and otherwise just return an empty tree. I'm not 100% sure this is the best approach, and it's different to how the original parser handles it.

Original desugar tree:

class <emptyTree><<C <root>>> < (::<todo sym>)
  def f<<todo method>>(x, &<blk>)
    <emptyTree>
  end
end

Original errors:

test/prism_regression/invalid/malformed_sig.rb:4: unexpected token tRCURLY https://srb.help/2001
     4 |sig,{params(x: Integer).void}
                                    ^

test/prism_regression/invalid/malformed_sig.rb:5: The method f does not have a sig https://srb.help/7017
     5 |def f(x)
        ^^^^^^^^
  Autocorrect: Use -a to autocorrect
    test/prism_regression/invalid/malformed_sig.rb:5: Insert extend T::Sig
    sig { params(x: T.untyped).returns(NilClass) }
     5 |def f(x)
        ^
Errors: 2

Prism desugar tree:

class <emptyTree><<C <root>>> < (::<todo sym>)
  <emptyTree>

  def f<<todo method>>(x, &<blk>)
    <emptyTree>
  end
end

Prism errors:

test/prism_regression/invalid/malformed_sig.rb:4: expected a `=>` between the hash key and value https://srb.help/2001
     4 |sig,{params(x: Integer).void}
                                    ^

test/prism_regression/invalid/malformed_sig.rb:4: unexpected '}'; expected a value in the hash literal https://srb.help/2001
     4 |sig,{params(x: Integer).void}
                                    ^

test/prism_regression/invalid/malformed_sig.rb:4: unexpected write target https://srb.help/2001
     4 |sig,{params(x: Integer).void}
            ^^^^^^^^^^^^^^^^^^^^^^^^^

test/prism_regression/invalid/malformed_sig.rb:4: unexpected write target https://srb.help/2001
     4 |sig,{params(x: Integer).void}
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

test/prism_regression/invalid/malformed_sig.rb:5: unexpected 'def', expecting end-of-input https://srb.help/2001
     5 |def f(x)
        ^^^

test/prism_regression/invalid/malformed_sig.rb:5: The method f does not have a sig https://srb.help/7017
     5 |def f(x)
        ^^^^^^^^
  Autocorrect: Use -a to autocorrect
    test/prism_regression/invalid/malformed_sig.rb:5: Insert extend T::Sig
    sig { params(x: T.untyped).returns(NilClass) }
     5 |def f(x)
        ^
Errors: 6