aacenc: Fix bug in LAME windowing where only one channel got initalized

I used the same loop counter for the inner and outer initalization loops.
This caused initalization to only run for the first channel. This in turn lead
to any channel other than the first using only short blocks.

Patch by Nathan Caldwell, saintdev at gmail

Originally committed as revision 25566 to svn://svn.ffmpeg.org/ffmpeg/trunk