Allocate bap and bap1 buffers using av_malloc().

This reduces the memory footprint when using less than 6 channels.
Modify bit allocation to swap the 2 buffers instead of using memcpy() and use
per-block pointers for bap. This is slightly faster (0.3%) in function
cbr_bit_allocation().

Originally committed as revision 26023 to svn://svn.ffmpeg.org/ffmpeg/trunk