Commit 31f9032b added the audio_preload feature; its goal is to
interleave audio earlier than the rest. Unfortunately, it has never ever
worked, because the check for whether a packet should be interleaved
before or after another packet was completely wrong: When audio_preload
vanishes, interleave_compare_dts returns 1 if the new packet should be
interleaved earlier than the packet it is compared with and that is what
the rest of the code expects. But the codepath used when audio_preload is
set does the opposite.

Also fixes potential undefined behaviour (namely signed integer
overflow).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>