bufferlist: make sure list is writable before adding or removing buffers
bufferlist: make sure list is writable before adding or removing buffers

https://bugzilla.gnome.org/show_bug.cgi?id=747439

krichter722/gstreamer,mparis/gstreamer,centricular/gstreamer,jpxiong/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,mparis/gstreamer,centricular/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,mparis/gstreamer