Frame Capture: Fix client buffers in MEC. In some cases we would neglect to reset the currently bound array buffer to "null" before capturing a client array buffer in MEC. Noticed in the "badland" trace context setup. Bug: angleproject:5133 Change-Id: I693edce763b449720a7f2123dcaf460854142e91 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3323683 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>