ssh_connect: treat libssh2 return code better

libssh2_knownhost_readfile() returns a negative value on error or
otherwise number of parsed known hosts - this was previously not
documented correctly in the libssh2 man page for the function.

Bug: http://curl.haxx.se/mail/lib-2011-02/0327.html
Reported by: murat

lib/ssh.c

@@ -2587,10 +2587,9 @@ static CURLcode ssh_connect(struct connectdata *conn, bool *done)
     rc = libssh2_knownhost_readfile(ssh->kh,
                                     data->set.str[STRING_SSH_KNOWNHOSTS],
                                     LIBSSH2_KNOWNHOST_FILE_OPENSSH);
-    if(rc) {
+    if(rc < 0)
       infof(data, "Failed to read known hosts from %s\n",
             data->set.str[STRING_SSH_KNOWNHOSTS]);
-    }
   }
 #endif /* HAVE_LIBSSH2_KNOWNHOST_API */