Ignore function node IDs not found when validating parent initializer calls

[ericglau]

Background:
When checking for issues in parent initializer calls according to #1095, we recursively check initializer functions and the functions that they call.

When an function calls a built-in function, the referenced AST node ID of the built in function may be negative (which was fixed in #1116), or it could be represented as a large positive number as reported in #1128.

We previously only ignored negative numbers, but since there is no way to conclusively identify that a large positive number is negative, we should also ignore when the referenced function node ID is not found.

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Testing:
I tested that this works by temporarily removing the checks for negative node IDs and allowing the new error handling to ignore the error. However, we should still keep the checks for negative node IDs for efficiency purposes (e.g. no need to dereference if we know that the id is negative and will not be found).

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Fixes #1128