d: Fix ICE in uda_attribute_p when looking up unknown attribute

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)

gcc/d/d-attribs.cc

@@ -216,10 +216,13 @@ uda_attribute_p (const char *name)
 	return true;
     }
 
-  for (const attribute_spec *p = targetm.attribute_table; p->name; p++)
+  if (targetm.attribute_table)
     {
-      if (get_identifier (p->name) == ident)
-	return true;
+      for (const attribute_spec *p = targetm.attribute_table; p->name; p++)
+	{
+	  if (get_identifier (p->name) == ident)
+	    return true;
+	}
     }
 
   return false;

gcc/testsuite/gdc.dg/pr95173.d

+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95173
+// { dg-do compile }
+// { dg-options "-Wattributes" }
+
+import gcc.attribute;
+
+@attribute("foo") // { dg-warning "unknown attribute .foo." }
+void f95173() 
+{
+}