fix bug: 'magit-run' always return "Git is already running"

If '*magit-process*' buffer is killed before magit-process finishes,
'magit-run' returns "Git is already running" error, always.
I fixed this. When '*magit-process*' buffer is killed, quit 'magit-process' force,
'magit-process-sentinel' and 'magit-process-filter' do nothing in buffer.