fetcher: Initialize output_stream_set_lock mutex
fetcher: Initialize output_stream_set_lock mutex

ostree pull-local crashed for me in thread_closure_unref () doing:
    g_mutex_clear (&thread_closure->output_stream_set_lock);

Seems like we never initialize this mutex.

Closes: #254
Approved by: cgwalters

gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree