buffer: Check return value of gst_memory_map()
buffer: Check return value of gst_memory_map()

Only do memory operations if the memory was succesfully map'ed

https://bugzilla.gnome.org/show_bug.cgi?id=728326

centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer