mem: Assert that the responderHadWritable is set only once

Change-Id: Ie3beeef25331f84a0a5bcc17f7a791f4a829695b
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-by: Stephan Diestelhorst <stephan.diestelhorst@arm.com>

src/mem/packet.hh

@@ -612,6 +612,7 @@ class Packet : public Printable
     void setResponderHadWritable()
     {
         assert(cacheResponding());
+        assert(!responderHadWritable());
         flags.set(RESPONDER_HAD_WRITABLE);
     }
     bool responderHadWritable() const