Fix segfault (null deref) in named initialisation of nested anonymous union

[el-remph]

This seems very specific but I did happen upon it organically. What were the chances? lol

Also adds a regression test, which causes previous chibicc version to deref NULL and segfault (the cc1 process dies with SIGSEGV, which causes the driver to exit(1)).

I think I'm right to assume that the only way mem->name can be NULL is if mem is an anonymous union/struct? Just wanted to make sure

[fuhsnn]

mem->name will be null for unnamed bitfields too, for example this will trigger the newly added assert:

struct { int : 7; int foo; } s = {.foo = 0};

IMO unnamed bitfields should be filtered out earlier though.

[el-remph]

@fuhsnn Thanks, I hadn't thought of that. Added that to the test, and fixed the code to allow it, and still prevent mem->name->len from being reached if mem->name is NULL