The target attribute table is not guaranteed to be set in all backends.

gcc/d/ChangeLog:

	PR d/95173
	* d-attribs.cc (uda_attribute_p): Don't search target attribute table
	if NULL.

gcc/testsuite/ChangeLog:

	PR d/95173
	* gdc.dg/pr95173.d: New test.

(cherry picked from commit 62e02c8729a75c4a859edc18e0bcafb87d717f46)