dh: Make sure we do not access uninitialized memory

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit ca62632170)

src/dh.c

@@ -1274,6 +1274,10 @@ int ssh_get_server_publickey(ssh_session session, ssh_key *key)
 
 ssh_key ssh_dh_get_current_server_publickey(ssh_session session)
 {
+    if (session->current_crypto == NULL) {
+        return NULL;
+    }
+
     return session->current_crypto->server_pubkey;
 }