Fix several issues with multistream argument validation.

As reported by Mark Warner opus_multistream_*_create were depending on
 the behavior of malloc(0) in order to correctly report some kinds of
 argument errors. Bad arguments could be incorrectly reported as
 allocation failures. This changes multistream to explicitly check the
 arguments like the single stream _create functions. The unit tests were
 enough to catch this on systems where malloc(0) returns NULL but didn't
 catch it on other systems because the later _init call would catch the
 bad arguments and trigger the correct error if and only if the malloc
 didn't return a null pointer.

In multistream_encoder_init failures of the internal non-multistream
 init calls were not being caught and propagated. Decode didn't have
 this problem. This propagates the errors and adds additional tests
 (the multistream encoder api is sill under tested) that would have
 detected this error.

Plus add some stronger tests for things like error==NULL for the _create
 functions that take a pointer for error output.