A change to Vulkan exposed this missing validation. In the Vulkan
back-end we do some caching on BeginXFB, which would perform an
invalid memory access. Adding the missing validation correctly
traps the error before we reach the back-end.

Bug: chromium:1171685
Bug: angleproject:4622
Change-Id: I5c92575a07149e431c4f260a4555ff196822c64e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2937022Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>