mem: Defer deletion of respQueue.front() in DRAMCtrl

The front() of respQueue was being deleted before the last usuage of
dram_pkt (which points to the same object) causing random crashes.

Change-Id: I89862d10599dc0d1a50717dac8ed9298b4d74a3d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28808
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/dram_ctrl.cc

@@ -716,7 +716,7 @@ DRAMCtrl::processRespondEvent()
         accessAndRespond(dram_pkt->pkt, frontendLatency + backendLatency);
     }
 
-    delete respQueue.front();
+    assert(respQueue.front() == dram_pkt);
     respQueue.pop_front();
 
     if (!respQueue.empty()) {
@@ -738,6 +738,8 @@ DRAMCtrl::processRespondEvent()
         }
     }
 
+    delete dram_pkt;
+
     // We have made a location in the queue available at this point,
     // so if there is a read that was forced to wait, retry now
     if (retryRdReq) {