Better handle missing nodes in parsing

[PgBiel]

I've fixed one case where the macro name might be missing due to an error node in #103, but it seems there are many more cases where nodes can just disappear and cause a crash. I managed to trigger a crash on enum parsing as well (line 46) by typing some invalid enum syntax (don't have a minimal example at the moment). Perhaps we should just assume that most nodes can be missing instead of trying to access right away.

[pherrymason]

This is interesting. I've arruved to the conclusióon that code in the parser should be more defensive indeed, as as you say, some nodes can be missing and that would cause crashes.