When declaring a polymorphic variable that is not a dummy, allocatable or
pointer, an ICE occurred due to a NULL pointer dereference.  Check for
that situation and punt.

gcc/fortran/
	PR fortran/93337
	* class.c (gfc_find_derived_vtab): Punt if name is not set.

(cherry picked from commit d9fb6f2b4f1321b059807ff6073156f07d9d376b)