[slang-tidy] Fix NoOldAlwaysSyntax checker segmentation fault

[likeamahoney]

By default LHSSymbol collector from ASTHelpers may store nullptr value when - assignment lhs is not lvalue directy (for example - {x} = 1;) and at another cases. Then it leads to segmentation fault at NoOldAlwaysSyntax checker when stored lhs value is dereferenced.

How to reproduce:

1. Create an bug.sv file:

module top();
    logic n;
    always @(*) begin
        {n} = 1;
    end
endmodule

2. Run NoOldAlwaysSyntax checker on it at any build. For example:

./build/gcc-11-release/bin/slang-tidy --config-file="Checks:-*, style-no-old-always-syntax"  bug.sv

[MikePopoloski]

Thanks for the PR!