multistream: improve arg check

Avoid undefined behavior (signed arithmetic overflow) or
implementation-defined behavior (malloc(0)) on out-of-range arguments,
e.g. opus_multistream_encoder_create(48000, 2, 2147483647, 1, ...)
or opus_multistream_surround_encoder_create(48000, 3, 0, ...).

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>