Otherwise glDrawElements(GL_TRIANGLES, -1, nullptr) would crash. This was
found by an ASSERT in ComputeIndexRange triggered by
dEQP-GLES2.functional.negative_api.vertex_array.draw_elements

BUG=

Change-Id: I5269031fa35aa6403c844561e04158361ee7950f
Reviewed-on: https://chromium-review.googlesource.com/292710Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>