Fix crash on zero size but non-NULL bufferData. Could cause a crash when running the WebGL CTS with Firefox. BUG=angle:675 Change-Id: I58bf9eed622660d4702b775f368ff9cbd693197a Reviewed-on: https://chromium-review.googlesource.com/203456 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>