When enabled, this would crash in debug because mFormat would be nullptr
on the target VkTexture, and the ASSERT would access mFormat via
getAspectFlags(). Fixed this by only calling getAspectFlags() if mFormat
is valid. I'm not sure this was the original reason this test was
disabled, but they pass now on SwiftShader.

Bug: angleproject:4092
Change-Id: I9b04bbc61dde14f722d3a510f3d38776f4f6c7ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2221420Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>