Fix GrMeshTest for Direct3D.

We were assuming that there was always a vertex buffer. However,
when doing instanced draws, there's not necessarily one.

Change-Id: I870aa039b5cc552dce2644f40b933de1f62ad80b
Bug: skia:9935
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301449
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed