Fix a possible crash in BN_from_montgomery_word

Thanks to Darovskikh Andrei for for reporting this issue.

Fixes: #5785
Fixes: #6302

Cherry-picked from f91e026e

 (without test/bntest.c)

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6310)