rtph264pay: fix potential crash when shutting down
rtph264pay: fix potential crash when shutting down

A race condition in the state change function may cause buffers
to be unreffed while they are still used by the streaming thread
in gst_rtp_h264_pay_send_sps_pps() resulting in a crash. Chain
up to the parent class first in the state change function to
make sure streaming has stopped and only then free those buffers.

https://bugzilla.gnome.org/show_bug.cgi?id=741381

jpakkane/gstreamer-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,jpakkane/gstreamer-plugins-good,ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good,jpakkane/gstreamer-plugins-good,jpakkane/gstreamer-plugins-good,wkatsak/gst-plugins-good,ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good