lavc: fix recoded subtitles end.

Text subtitles packets are not 0-terminated (and if they are,
it is handled by the recoding process since 0 is a valid
Unicode code point). The terminating 0 would overwrite the
last payload octet.

OTOH, packets must be 0-padded.

Fix a problem reported in trac ticket #2431.
dpxdec: Fix decoding of RGBA10

Fixes Ticket2392

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>