GrGLGpu: Avoid potential nullptr dereference

We were checking if 'mipMapsStatus' was nullptr, and if it was,
our 'else' clause would have us always dereference this nullptr.
We change the logic to avoid a dereference in this case.

Change-Id: I2bc091c4de0c9b62ae733026cea3be37442820eb
Reviewed-on: https://skia-review.googlesource.com/c/191323
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed