aac: Keep decode_band_types() from eating all padding at the end of a buffer.

Due to a shortcoming in the AAC specification, if an all zero buffer is
fed to section data decoding it will never terminate. That means without
a buffer exhaustion check decode_band_types() will consume all input
buffer padding. Worse if a get_bits() implementation that returns zeros
when padding is exhausted is used, the function will never terminate.

The fixes that by added a buffer exhaustion check in the sectioning
decoding loop.

Originally committed as revision 22044 to svn://svn.ffmpeg.org/ffmpeg/trunk
