In some cases, binding nullptr could query for the internal format
of the null object, causing us to dereference null and crash.

BUG=angleproject:1080
TEST=dEQP-GLES3.functional.multisample.fbo_*

Change-Id: I592ab31dc1efadfac003c39051632fc2a0fff0cc
Reviewed-on: https://chromium-review.googlesource.com/286853Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>