serial: reject new commands when the port is closed.

Otherwise, we can schedule a main loop call to
mm_serial_port_queue_process() for an object that's about to
disappear, leading to a crash.

BUG=chromium-os:21169
TEST=Repeatedly run "start modemmanager" and "stop modemmanager" and
watch logs for a crash.
Change-Id: I433a76855c52536eb2b99a5ecf26ac71afe1f8bb
Reviewed-on: https://gerrit.chromium.org/gerrit/11217
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Tested-by: Nathan J. Williams <njw@chromium.org>
Commit-Ready: Nathan J. Williams <njw@chromium.org>
1 file changed