Binary searching would hang if the fragment items do NOT have timestamp for the
specified stream.

For example, a fmp4 consists of separated 'moof' boxes for each track, and
separated 'sidx' for each segment, but no 'mfra' box.  Then every fragment item
only have the timestamp for one of its tracks.

Example:
ffmpeg -f lavfi -i testsrc -f lavfi -i sine -movflags dash+frag_keyframe+skip_trailer+separate_moof -t 1 out.mp4
ffmpeg -ss 0.5 -i out.mp4 -f null none

Also fixes the hang in ticket #7572, but not the reason for having
AV_NOPTS_VALUE timestamps there.
Signed-off-by: Charles Liu <liuchh83@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>