Don't refer to allocation map entry after deallocating it

debugserver's MachTask::DeallocateMemory when removing an
allocate entry from our map (in resposne to an '_m' packet),
copy the size from the entry before removing it from the
map and then using the iterator to fix an ASAN error on
the bots when running TestGdbRemoteMemoryAllocation.py

rdar://76595998