mem-cache: Avoid calling .front() on a possibly empty std::list

In the call to MSHR::promoteWritable() the deferredTargets list can be
empty, so we should check that case before calling .front(). The new logic
matches MSHR::promoteReadable().

Change-Id: Ic1d05e42f32b2c02226ca88d2155225f592f667f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57249
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/cache/mshr.cc

@@ -678,6 +678,9 @@ MSHR::promoteReadable()
 void
 MSHR::promoteWritable()
 {
+    if (deferredTargets.empty()) {
+        return;
+    }
     PacketPtr def_tgt_pkt = deferredTargets.front().pkt;
     if (deferredTargets.needsWritable &&
         !(hasPostInvalidate() || hasPostDowngrade()) &&