crypto/ecdh: explicitly reject mismatched curves in ECDH

crypto/ecdh: explicitly reject mismatched curves in ECDH

Return an explicit error when PrivateKey.ECDH is called with a PublicKey
which uses a different Curve. Also document this requirement, even
though it is perhaps obvious.

Fixes #58131

Change-Id: I739181a3f1283bed14fb5ee7eb78658b854d28d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/464335
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>