c++: Fix ICE in check_local_shadow with enum [PR95560]

c++: Fix ICE in check_local_shadow with enum [PR95560]

Another indication that perhaps this warning is emitted too early.  We
crash because same_type_p gets a null type: we have an enumerator
without a fixed underlying type and finish_enum_value_list hasn't yet
run.  So check if the type is null before calling same_type_p.

	PR c++/95560
	* name-lookup.c (check_local_shadow): Check if types are
	non-null before calling same_type_p.

	* g++.dg/warn/Wshadow-local-3.C: New test.