client session BUGFIX double free

Fixes cesnet/netopeer2#454
diff --git a/src/session_client_ssh.c b/src/session_client_ssh.c
index a2f3dbf..8f76e49 100644
--- a/src/session_client_ssh.c
+++ b/src/session_client_ssh.c
@@ -1845,7 +1845,7 @@
 
     session = _nc_connect_libssh(sess, ctx, &ssh_ch_opts, timeout);
     if (!session) {
-        ssh_free(sess);
+        /* sess is freed */
         return NULL;
     }