Explicitly fail read/writePixels in invalid color space scenarios

Explicitly fail read/writePixels in invalid color space scenarios

It's not well defined what to do when moving from a nullptr color space to
a tagged destination (drawing, reading, writing, etc...). In these
scenarios, at least, we can choose to disallow the operation (rather than
produce an unexpected or inconsistent result).

BUG=skia:

Change-Id: I033b23c6f2bb00664efc8fdab1b3f52053d77695
Reviewed-on: https://skia-review.googlesource.com/6600
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>