Fix ETC->DXT small mip transcode crash. BUG=angleproject:1510 Change-Id: I42355607235a05fe276342084acd1eb359c16de4 Reviewed-on: https://chromium-review.googlesource.com/390414 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>