In some cases, where the user would create a buffer with NULL (empty)
data, we would attempt to dereference NULL when drawing with the
buffer as a vertex attribute.

BUG=angle:749

Change-Id: Ied5ecbab4608c85890cdf7cc32a8dae46989e33b
Reviewed-on: https://chromium-review.googlesource.com/219090Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>