This stops rejecting 0-byte packets, adds checks everywhere we
access the first byte of a packet, updates the th_decode_packetin()
return value checking so that duplicate frames aren't thrown away,
and updates video pts generation so the resulting frames are
displayed correctly,

Fixes bug #3416.
Signed-off-by: Rafaël Carré <funman@videolan.org>
(cherry picked from commit 9c7198ea)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>