lavc/tta: Use a safer check for encrypted streams

The user can provide a password even when the stream
is not encrypted, so check the value of s->format
instead of s->pass in ttafilter_init().

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Changelog: Add entry for encrypted TTA stream decoding

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>