Mem: fix master id assertion in cache_impl.hh

The assertion was applied to the wrong packet.
This patch fixes the issue rerported by Xiang Jiang on the gem5-dev mailing list.

src/mem/cache/cache_impl.hh

@@ -903,7 +903,7 @@ Cache<TagStore>::handleResponse(PacketPtr pkt)
 
                 assert(!target->pkt->req->isUncacheable());
 
-                assert(pkt->req->masterId() < system->maxMasters());
+                assert(target->pkt->req->masterId() < system->maxMasters());
                 missLatency[target->pkt->cmdToIndex()][target->pkt->req->masterId()] +=
                     completion_time - target->recvTime;
             } else if (pkt->cmd == MemCmd::UpgradeFailResp) {