Avoid assertion in MSAA rendering buffer about indexed draw with zero index count

Avoid assertion in MSAA rendering buffer about indexed draw with zero index count

Also add some GrMesh::validate() calls earlier to make future debugging easier. Previously this only asserted when the draws executed, now it asserts as soon as the draw is prepared in the op subclass.

Bug: skia:
Change-Id: Ibdd4488b9a9263f0ed337a2c8c6066bc36297001
Reviewed-on: https://skia-review.googlesource.com/16029
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>