rtpengine: Fix extra process in pkg.stats

When DTMF events socket is not configured, pkg.stats RPC method returns
an empty entry.
{
        entry: 13
        pid: 0
        rank: -128
        used: 0
        free: 0
        real_used: 0
        total_size: 67108864
        total_frags: 0
        desc:
}

Fixes: 26f6e57c8f ("Register a new worker process for dtmf event listener")

src/modules/rtpengine/rtpengine.c

@@ -2234,8 +2234,10 @@ static int mod_init(void)
 			}
 		}
 
-		register_procs(1);
-		cfg_register_child(1);
+		if(rtpengine_dtmf_event_sock.len > 0) {
+			register_procs(1);
+			cfg_register_child(1);
+		}
 	}
 
 	return 0;