[canvaskit] Fix null dereference when GrContext cannot be made

This happens when validation fails, for example.

Change-Id: Idbd552fbce51c5cf1543fc7a0a34a87230264d6e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253658
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed