Fixes:
 * Crash in conformance/extensions/oes-vertex-array-object.html

BUG=angleproject:880

Change-Id: I90eba9b26b2834644ce3ba0e78cab3417c2a7bd1
Reviewed-on: https://chromium-review.googlesource.com/273325Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>