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.