avformat/mpegts: pass MpegTSContext ptr explicitly (fixes #3721)

AVFormatContext->priv_data is not always a MpegTSContext, it can be
RTSPState when decoding a RTP stream. So it is necessary to pass
MpegTSContext pointer explicitly.

This fixes memory corruption from bug #3721 (RTSPState is smaller than
MpegTSContext thus innocent memory gets overwritten).

Signed-off-by: Alexander V. Lukyanov <lavv17f@gmail.com>
Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
