v4l2bufferpool: unref pool when v4l2_allocator_new() fails

https://bugzilla.gnome.org/show_bug.cgi?id=739791

sys/v4l2/gstv4l2bufferpool.c

@@ -1489,6 +1489,7 @@ dup_failed:
 allocator_failed:
   {
     GST_ERROR_OBJECT (pool, "Failed to create V4L2 allocator");
+    gst_object_unref (pool);
     return NULL;
   }
 }