Fix crash switching to Vulkan in viewer on Windows

BUG=skia:

Change-Id: I626aa6a1571311600a6208c42a8c3e9509d037de
Reviewed-on: https://skia-review.googlesource.com/9078
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed