upstream: Fix BN_is_prime_* calls in SSH, the API returns -1 on

error.

Found thanks to BoringSSL's commit 53409ee3d7595ed37da472bc73b010cd2c8a5ffd
by David Benjamin.

ok djm, dtucker

OpenBSD-Commit-ID: 1ee832be3c44b1337f76b8562ec6d203f3b072f8
