userauth: check for too large userauth_kybd_auth_name_len (#650)
userauth: check for too large userauth_kybd_auth_name_len (#650)

... before using it.

Reported-by: MarcoPoloPie
Fixes #649
libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2