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

src/libostree/ostree-fetcher.c

@@ -541,6 +541,7 @@ _ostree_fetcher_constructed (GObject *object)
   self->thread_closure->output_stream_set = g_hash_table_new_full (NULL, NULL,
                                                                    (GDestroyNotify) NULL,
                                                                    (GDestroyNotify) g_object_unref);
+  g_mutex_init (&self->thread_closure->output_stream_set_lock);
 
   if (g_getenv ("OSTREE_DEBUG_HTTP"))
     {