Small tweak to ec_GFp_simple_points_make_affine.

(Imported from upstream's 267e6f3cc0ef78dea4e5cf93907a71556a45f008)

Change-Id: I99cfd0196b9625449c50494562c44f57f09fed17
Reviewed-on: https://boringssl-review.googlesource.com/2167
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>