Fix ASTDereferencerError when validating initializers

[ericglau]

Referenced AST ids can be negative if they refer to nodes that are not in the solc output. For example, require statements seem to have referenced AST ids of -18.

When validating initializers, we should ignore negative AST id references.

Fixes #1116

[ckartik]

When is this expected to get in, and are there any recommendations to get around this issue, we're currently running into build issues on our CI because of this.

[ckartik]

We're running into the following erros:

/home/runner/.npm/_npx/fb3494f8bab08a4d/node_modules/solidity-ast/dist/ast-dereferencer.js:73
        throw new ASTDereferencerError(id, nodeType);
              ^

ASTDereferencerError: No node with id -18 of type FunctionDefinition