Vulkan: Fix validation of MSRTT framebuffer attachment The attachment was being accessed before being validated. A fuzzer test was giving this function texture ID -1. Bug: chromium:1146753 Change-Id: Ic48f5fd2eed4252e90be5d63fd698b38bd3ef85f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2623814 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>