It's not safe to raise an exception in `PyObject_ClearWeakRefs()` if one
is not already set, since it may be called by `_Py_Dealloc()` and hit
https://github.com/python/cpython/blob/5a90de0d4cbc151a6deea36a27eb81b192410e56/Objects/object.c#L2843-L2860.

Additionally, make sure we clear the weakrefs even when tuple allocation
fails.

This bug predates pythongh-111926. If it's getting tickled now, I suspect it's
because we always allocate a tuple when clearing weakrefs that have callbacks.

Depending on finalization, we may encounter another allocation failure
that causes the interpreter to exit with a non-zero status. That's OK,
we just need to verify that we're not exiting because weakref clearing
raised an exception.