Fix infinite writebacks bug in cache.

src/mem/cache/cache_impl.hh:
    Make sure to pop the list.  Fixes infinite writeback bug.
src/mem/cache/miss/mshr_queue.cc:
    Add an assert as sanity check in case .full() stops working again.

--HG--
extra : convert_revision : d847e49a397eeb0b7c5ac060fcfc3eaeac921311

src/mem/cache/cache_impl.hh

@@ -326,6 +326,7 @@ Cache<TagStore,Buffering,Coherence>::handleResponse(Packet * &pkt)
                                    writebacks, pkt);
             while (!writebacks.empty()) {
                     missQueue->doWriteback(writebacks.front());
+                    writebacks.pop_front();
             }
         }
         missQueue->handleResponse(pkt, curTick + hitLatency);