Fix null pointer dereference in glGetBufferPointerv BUG=angle:687 Change-Id: If8f215d4a86332b9e6e791263bdeeb44b23a42a2 Reviewed-on: https://chromium-review.googlesource.com/205711 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Shannon Woods <shannonwoods@chromium.org>