avformat/hlsenc: process hls_time value too small sence

avformat/hlsenc: process hls_time value too small sence

The target duration will be a negative value when there are
some b frames after prevous frame, the pts after current packet
is large than the pts of current packet, so the target duration
will compute as 0.040000 - 0.080000, then the value of the target
duration will be -0.040000. so hls muxer should check the pts after
current packet minus the pts of current packet, hls muxer can split
the stream as a segment if the target duration is neither negative nor
zero, hls muxer cannot split the stream as a segment if the
target duration is either negative or zero then get the next packet
until the target duration is not negative or zero.

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
Suggested-by: Zhili Zhao <quinkblack@foxmail.com>
Signed-off-by: liuqi05 <liuqi05@kuaishou.com>