Fix 0-byte Ogg packet handling.

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>