Fix crash in C# bindings generator with bad enum documentation XML

[aaronfranke]

I had a bad [enum ...] in my documentation XML. This was crashing the C# bindings generator, because the code was running p_target_itype->is_intentionally_ignored without checking if p_target_itype is null or not.

Note that these functions have code paths for dealing with a null p_target_itype, so it seems to not be a case where we can simply have guard clauses. In most of the functions calling is_intentionally_ignored the code already checked for a null p_target_itype and used a different code path, but this was missing only for the error case of enums.

I've marked this as cherrypick for 4.3 and 4.4. The bug seems to also exist in 4.2 and earlier but the code is a bit different so the cherrypick can't be done automatically, and I don't know if it's worth the effort for such an old branch.

[raulsntos]

Looks good to me. Thanks!

[Repiteo]

Thanks!