avformat/mpegenc: improve field names in underflow error message

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegenc: change assert to av_assert0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegenc: Better default for max_delay

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegenc: Better heuristic for ordering packets

This prevents underflows in quite constrained cases

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