gvfschannel: do not crash when channel is blocked

Backend could crash, when channel is blocked and got new request
unless current job is set. Therefor send_reply_cb is called
without current job and cause following error:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7de12a5 in g_vfs_job_emit_finished (job=0x0) at gvfsjob.c:322
322      g_assert (!job->finished);

To fix the problem be sure error job is set.

https://bugzilla.gnome.org/show_bug.cgi?id=737842
gvfschannel: do not call close job if channel is blocked

We don't want to call new jobs when backend is force unmounted.

https://bugzilla.gnome.org/show_bug.cgi?id=737842
gvfschannel: don't abort when finalizing

Remove g_assert (channel->priv->backend_handle == NULL), because it
could happen when backend is force unmounted.

https://bugzilla.gnome.org/show_bug.cgi?id=737842