MESI CMP: Unset TBE pointer in L2 cache controller

The TBE pointer in the MESI CMP implementation was not being set to NULL
when the TBE is deallocated. This resulted in segmentation fault on testing
the protocol when the ProtocolTrace was switched on.

src/mem/protocol/MESI_CMP_directory-L2cache.sm

@@ -593,6 +593,7 @@ machine(L2Cache, "MESI Directory L2 Cache CMP")
 
   action(s_deallocateTBE, "s", desc="Deallocate external TBE") {
     L2_TBEs.deallocate(address);
+    unset_tbe();
   }
 
   action(jj_popL1RequestQueue, "\j", desc="Pop incoming L1 request queue") {