Fix timing leak in BN_from_montgomery_word.

BN_from_montgomery_word doesn't have a constant memory access pattern.
Replace the pointer trick with a constant-time select. There is, of
course, still the bn_correct_top leak pervasive in BIGNUM itself.

See also https://boringssl-review.googlesource.com/22904 from BoringSSL.

(backport from f345b1f3

 signed off by
David Benjamin <davidben@google.com>)

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>