gh-111178: fix UBSan failures for gdbmobject

[picnixz]

Just for something I thought of when I was doing those UBSan tasks:

• for static types, I can add an assert on the type.
• for dynamic types, I need the state if I want my assert.

AFAIR, I skipped the assert when the type is internal and dynamic (namely I used a simple pointer cast). For now, I suggest keeping the macro form since it wouldn't really matter if we just have a static inline function without the assertion check.

I've also suppressed an unused return value just for my IDE to be happy but I can revert the commit if needed.

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178