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/+/2623814Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>