cache: fix longstanding prefetcher bug

Thanks to Joe Gross for pointing this out (again?).
Apologies to anyone who pointed it out earlier and
we didn't listen.

src/mem/cache/cache_impl.hh

@@ -1308,7 +1308,7 @@ Cache<TagStore>::getNextMSHR()
 
     // fall through... no pending requests.  Try a prefetch.
     assert(!miss_mshr && !write_mshr);
-    if (!mshrQueue.isFull()) {
+    if (prefetcher && !mshrQueue.isFull()) {
         // If we have a miss queue slot, we can try a prefetch
         PacketPtr pkt = prefetcher->getPacket();
         if (pkt) {