Fix MJPEG decoder for AMV files.

Fix MJPEG decoder for AMV files.
Since decoding is doing from the end and aligned by 16
previous code worked correctly only when picture height was dividable by 16,
otherwise it provides garbage in top lines and truncates bottom.
New code adjusts data[] pointers taking in account alignment issue.

Originally committed as revision 10727 to svn://svn.ffmpeg.org/ffmpeg/trunk