tls: fix coverity warning

Type: fix

Avoid complaint that we're potentially incrementing ii which could be
null.

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I8511c07b1c2f260cc0e526d9aefeb4a051d98edf

src/plugins/tlsopenssl/tls_async.c

@@ -417,7 +417,7 @@ openssl_async_node_enable_disable (u8 is_en)
 
   foreach_vlib_main ()
     {
-      if (have_workers && ii != 0)
+      if (have_workers && this_vlib_main->thread_index)
 	{
 	  vlib_node_set_state (this_vlib_main, tls_async_process_node.index,
 			       state);