go/parser: `goto;` is incorrectly valid

While poking around the `go/types` internals, i noticed that nothing really "type-checks" `goto`s without labels.

First pass:
https://github.com/golang/go/blob/500675a7c8c72bd6b1054a7eb4daaf61970f5ad7/src/go/types/stmt.go#L578-L608

Second pass:
https://github.com/golang/go/blob/500675a7c8c72bd6b1054a7eb4daaf61970f5ad7/src/go/types/labels.go#L173-L176


Also it also turns out that:

```go
func test() {
    goto;
}
```

Is accepted by the `go/parser`. The spec disallows `goto`s without labels, so we should either return an appropriate error in `go/types` or `go/parser`. Currently  `go/types` returns an `InvalidSyntaxTree` error.

https://github.com/golang/go/blob/500675a7c8c72bd6b1054a7eb4daaf61970f5ad7/src/go/types/stmt.go#L605-L607

CC @griesemer @adonovan


	case *ast.BranchStmt:
	if s.Label != nil {
	check.hasLabel = true
	return // checked in 2nd pass (check.labels)
	}
	switch s.Tok {
	case token.BREAK:
	if ctxt&breakOk == 0 {
	check.error(s, MisplacedBreak, "break not in for, switch, or select statement")
	}
	case token.CONTINUE:
	if ctxt&continueOk == 0 {
	check.error(s, MisplacedContinue, "continue not in for statement")
	}
	case token.FALLTHROUGH:
	if ctxt&fallthroughOk == 0 {
	var msg string
	switch {
	case ctxt&finalSwitchCase != 0:
	msg = "cannot fallthrough final case in switch"
	case ctxt&inTypeSwitch != 0:
	msg = "cannot fallthrough in type switch"
	default:
	msg = "fallthrough statement out of place"
	}
	check.error(s, MisplacedFallthrough, msg)
	}
	default:
	check.errorf(s, InvalidSyntaxTree, "branch statement: %s", s.Tok)
	}

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go/parser: `goto;` is incorrectly valid #70957

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	case *ast.BranchStmt:
	if s.Label == nil {
	return // checked in 1st pass (check.stmt)
	}

go/parser: goto; is incorrectly valid #70957

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