ada: Compiler hangs on invalid postcondition

In some cases involving an illegal reference to F'Result in
the postcondition for a function not named F, the compiler would
hang instead of correctly diagnosing the error.

gcc/ada/

	* sem_attr.adb (Denote_Same_Function): Handle the case where
	Has_Homonym (Pref_Id) returns True but Homonym (Pref_Id) returns
	an empty result.

gcc/ada/sem_attr.adb

@@ -5921,7 +5921,9 @@ package body Sem_Attr is
             --  When a qualified name is used for the prefix, homonyms may come
             --  before the current function in the homonym chain.
 
-            elsif Has_Homonym (Pref_Id) then
+            elsif Has_Homonym (Pref_Id)
+              and then Present (Homonym (Pref_Id))
+            then
                return Denote_Same_Function (Homonym (Pref_Id), Spec_Id);
             end if;