aaclatm: Eliminate dummy packets due to muxlength calculation.
Muxlength does not include the 3 bytes of AudioSyncStream() before the
AudioMuxElement(). If these three bytes are not accounted for the last three
bytes of the LATM packet are sent back to the decoder again.
Originally committed as revision 25685 to svn://svn.ffmpeg.org/ffmpeg/trunk