Although the manpage says that BIGNUM functions return 0 on error,
OpenSSL versions before 1.0.2n and current LibreSSL versions may return
-1 instead.
Note that the implementation of OpenSSL::BN#mod_inverse is extracted
from BIGNUM_2c() macro as it didn't really share the same function
signature with others.
https://github.com/ruby/openssl/commit/9b59f34345
