Since the loop always goes up to MAX_VERTEX_ATTRIBS, the ASSERTION may be
triggered even though no more attributes are enabled.

BUG=angle:740

Change-Id: I2bb30d7e9f426ae1a0b8250e65c1fd02107973ac
Reviewed-on: https://chromium-review.googlesource.com/217027Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>