usbtty: avoid potential NULL pointer dereference

If current_urb is NULL it should not be dereferenced.

The problem was indicated by cppcheck.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>

drivers/serial/usbtty.c

@@ -850,6 +850,13 @@ static int write_buffer (circbuf_t * buf)
 	struct urb *current_urb = NULL;
 
 	current_urb = next_urb (device_instance, endpoint);
+
+	if (!current_urb) {
+		TTYERR ("current_urb is NULL, buf->size %d\n",
+		buf->size);
+		return 0;
+	}
+
 	/* TX data still exists - send it now
 	 */
 	if(endpoint->sent < current_urb->actual_length){
@@ -871,12 +878,6 @@ static int write_buffer (circbuf_t * buf)
 		 */
 		while (buf->size > 0) {
 
-			if (!current_urb) {
-				TTYERR ("current_urb is NULL, buf->size %d\n",
-					buf->size);
-				return total;
-			}
-
 			dest = (char*)current_urb->buffer +
 				current_urb->actual_length;