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>
1 file changed