glCompressedTexImage3D was missing a check that the format was valid
before trying to compute the expected data size.  This lead to an
assertion failure in the size computation code.

BUG=602737

Change-Id: I74e6ced082e78396a0af92317fb1c0098299a026
Reviewed-on: https://chromium-review.googlesource.com/441185Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>