vaapivideobufferpool: fix leak of created allocator

Since it's created by itself, it should be unref-counted
after gst_buffer_pool_config_set_allocator call. Afterwards,
this allocator will be ref-counted again when assigning to priv->allocator.

https://bugzilla.gnome.org/show_bug.cgi?id=781577

gst/vaapi/gstvaapivideobufferpool.c

@@ -179,6 +179,7 @@ gst_vaapi_video_buffer_pool_set_config (GstBufferPool * pool,
       allocator = gst_vaapi_video_allocator_new (priv->display, &new_vip,
           surface_alloc_flags, 0);
       gst_buffer_pool_config_set_allocator (config, allocator, NULL);
+      gst_object_unref (allocator);
     }
   }