Detect and prevent reading over the end of counts_*. We pass the error
through a context variable as this is simpler and i think also faster, but
the return value of functions could be used instead of course.
The code also ensures as a side effect that the AC decoder state does not
become invalid.
This fixes all known crashes. And outputs nothing in case of an error instead
of random noise.

Originally committed as revision 12316 to svn://svn.ffmpeg.org/ffmpeg/trunk
