Capture/Replay: Avoid null pointer dereference in MEC Small fix to progress Manhattan MEC. Bug: angleproject:4091 Test: Working through Manhattan frames 5-10, TRex still works Change-Id: I7981e19a64f121b2ecca4451574ca2886641c6ac Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2039532 Commit-Queue: Cody Northrop <cnorthrop@google.com> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>