opus

Remove large multistream stack buffers.

This avoids allocating any buffers on the stack that depend on the
 total channel count.
Such buffers could easily exceed the size of the
 NONTHREADSAFE_PSEUDOSTACK.
It also checks the frame_size argument in both the encoder and
 decoder to avoid allocating large stack buffers for opus_encode()
 calls that would fail anyway or opus_decode() calls that would
 never use all that space anyway.