Vulkan: Minor fix in "WindowSurfaceVk::hasStagedUpdates()". This CL adds check to prevent checking buffer that is not technically acquired. Also if in the future this function will be called for double buffered surfaces it will not check a previous buffer. Bug: b/237449314 Change-Id: I79f1a2c4cd05a5424e963503891449f310177e93 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4262070 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>