mediamanager: prevent deadlock when reading form appdata source

gst_app_sink_pull_sample() blocks until there is a sample available, but
when it arrives, gst_app_sink_render() invokes callback
appsink_new_sample(), which wants to lock appdata_mutex already held by
purple_media_manager_receive_application_data() in the main thread.

To prevent the deadlock we now check there actually are some samples
available before calling gst_app_sink_pull_sample().