make append_gamut_tranform() never fail

make append_gamut_tranform() never fail

The only way it could ostensibly fail is if we get non-XYZ color
spaces, which should just not happen.  Assert that doesn't happen
and safely do nothing instead of failing.

This is one of the leaf nodes to getting SkCreateRasterPipelineBlitter
to never fail.  Next come SkColorFilter:: and SkShader::appendStages().

Change-Id: I5c7a8c63d0a9837e2e55208e1674796d86f45307
Reviewed-on: https://skia-review.googlesource.com/16002
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>