Always clamp curve25519 keys prior to use.

This fixes an issue where a private key contained bits that should
have been cleared by the clamping process, but were passed through
to the scalar multiplication routine and resulted in an invalid
public key.

Issue diagnosed (and an initial fix proposed) by shamaz.mazum in
PR 252894.

This fix suggested by Jason Donenfeld.

PR: 252894
Reported by: shamaz.mazum
Reviewed by: dch
MFC after: 3 days