wrapper: Avoid memory leak on errors during key exchange

As reported by oss-fuzz

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28075

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 9c6404aa49)

src/wrapper.c

@@ -182,6 +182,7 @@ void crypto_free(struct ssh_crypto_struct *crypto)
         crypto->ecdh_privkey = NULL;
     }
 #endif
+    SAFE_FREE(crypto->dh_server_signature);
     if (crypto->session_id != NULL) {
         explicit_bzero(crypto->session_id, crypto->digest_len);
         SAFE_FREE(crypto->session_id);