libANGLE: Assert on mExecutable when syncing images In the loop in syncImagesInit mExecutable is dereferenced and mProgram is not even touched, so assert on mExecutable Bug: angleproject:6557 Change-Id: Icf482bda281c574e952113a97cbaf1e1d93fc1e3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3220631 Commit-Queue: Gert Wollny <gert.wollny@collabora.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>