Don't attempt to stop the receive thread if we never started it.

If socket initialization fails, DhcpState#exit will call
mReceiveThread#stop and crash the system with an NPE. Make sure
we don't do that if mReceiveThread is null, and properly null it
out when exiting.

Bug: 23088314
Change-Id: I4378d8280f9d8588f5eaa8bd7ade61beab1c3ce2