Left shift of a negative number is undefined behaviour

Fix undefined behaviour in curve25519.c. Prior to this running with
ubsan produces errors like this:

crypto/ec/curve25519.c:3871:18: runtime error: left shift of negative
value -22867

[extended tests]

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3600)