avformat/ac3dec: Fix to prevent runaway ac3 detection by looking at the actual frame rather than the first detected frame.

When detecting a swapped AC3 marker the data of the frame is swapped. However, in subsequent frames the data swapped is taken from the first frame rather than the current frame.
Signed-off-by: Marijn Meijles <marijn@bitpit.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>