c++: ICE with -fsanitize=vptr and constexpr dynamic_cast [PR98103]

c++: ICE with -fsanitize=vptr and constexpr dynamic_cast [PR98103]

-fsanitize=vptr initializes all vtable pointers to null so that it can
catch invalid calls; see cp_ubsan_maybe_initialize_vtbl_ptrs.  That
means that evaluating a vtable reference can produce a null pointer
in this mode, so cxx_eval_dynamic_cast_fn should check that and give
and error.

gcc/cp/ChangeLog:

	PR c++/98103
	* constexpr.c (cxx_eval_dynamic_cast_fn): If the evaluating of vtable
	yields a null pointer, give an error and return.  Use objtype.

gcc/testsuite/ChangeLog:

	PR c++/98103
	* g++.dg/ubsan/vptr-18.C: New test.