Fix crash in ~GrContext

BUG=804352, 804111
Change-Id: Ia8475a4a8a6de77438cde7df53931dc70c405b16
Reviewed-on: https://skia-review.googlesource.com/98341
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed