The AAC frame header uses 13 bits for the frame size, so the buffer should

The AAC frame header uses 13 bits for the frame size, so the buffer should
have room for such a frame.  A frame that large seems unlikely in a valid
file, but leaving room for it spares us the need to check it elsewhere.
Moving the buffer to the end of the struct made debugging this easier.

Originally committed as revision 5487 to svn://svn.ffmpeg.org/ffmpeg/trunk