Add nullptr checking for rename logging

[henryhchchc]

There is a segmentation fault during a rename when the position is not pointing to a valid symbol. Add a checking to avoid that.

POC:

function id(uint x) pure returns (uint) {
    return x;
}

function zero(uint) pure returns (uint) {
    return 0;
}

contract C {
    function f(uint z) pure external returns(uint) {
        return z.id();
    }

    function g(uint z) pure external returns (uint) {
        return z.zero();
    }

    using {id, zero} for uint;
}

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "textDocument/rename",
  "params": {
    "textDocument": {
      "uri": "file:///tmp/export/input_2/workspace/main.sol"
    },
    "position": {
      "line": 10,
      "character": 20
    },
    "newName": "pure"
  }
}

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[clonker]

Thanks!