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