websocket: Check frame->wsc in ws_frame_transmit for NULL.

Add frame.wsc == NULL check in ws_frame_transmit similar to the checks
in ws_close and friends and ws_frame_receive to avoid crashing if the
tcp connection was been terminated prematurely.

(cherry picked from commit c109755d347500d8dcfa92a75cb11c9cea2727c2)

modules/websocket/ws_frame.c

@@ -743,6 +743,11 @@ int ws_frame_transmit(void *data)
 	frame.payload_len = wsev->len;
 	frame.payload_data = wsev->buf;
 	frame.wsc = wsconn_get(wsev->id);
+	if (frame.wsc == NULL)
+	{
+		LM_ERR("WebSocket outbound connection not found\n");
+		return -1;
+	}
 
 	LM_DBG("Tx message:\n%.*s\n", frame.payload_len,
 			frame.payload_data);