Make sure GrBackendTextures are valid before starting to do things with them

Bug: skia:
Change-Id: If05706f0fd732bb1b2bfb788b540e795fa724814
Reviewed-on: https://skia-review.googlesource.com/119887
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed