Fix texture buffer validation w.r.t to non-existing buffer
Fix texture buffer validation w.r.t to non-existing buffer

Bug: angleproject:3573
Test: dEQP.KHR_GLES31/core_texture_buffer_texture_buffer_errors
Change-Id: I031abd94d6c06e539a2a89a82f2af604a6f3b5b8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2533493
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Jonah Ryan-Davis <682bdb9e7d8ba359ef169b8db374e7a99d394a46@google.com>
Commit-Queue: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle