https://go-review.googlesource.com/#/c/2421/ contains an unfortunate
slip where IsOnCurve is called on the CurveParams rather than the curve.
This doesn't really matter, but it's a pain for people doing tricks with
crypto/elliptic and means that 1.5 would be a regression for them
without this change.

See https://groups.google.com/forum/#!topic/golang-dev/i8OPUTYctOk

Change-Id: Ifa5f25f9a95d7484cb53d4883cfd78dc58a0f9a7
Reviewed-on: https://go-review.googlesource.com/12506Reviewed-by: Russ Cox <rsc@golang.org>