avformat/mpegtsenc: move startcode validity check to ff_check_h264_startcode

Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/wtvenc: advise user when H264 startcode is not present

Fixes ticket #3473.

Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>