Fix compilation with assert turned off

git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@4100 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/datasrc/memory_datasrc.cc

@@ -175,6 +175,7 @@ struct MemoryZone::MemoryZoneImpl {
         // This case should not happen because we enable callback only
         // when we add an RR searched for above.
         assert(0);
+        return (false);
     }
 
     // Implementation of MemoryZone::find