avformat/mpegts: Ensure that mpegts_get_dts() only considers packets at or after the given position

This fixes an infinite loop
An alternative fix would be to revert d73cbc22c5f2c305838d9867538cf4ad59c4035f
but that would worsen error resilience.

Found-by: Justin Ruggles
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
