USB: serial: add special case for processing of empty read urbs

Return immediately from generic process_read_urb if urb is empty.

Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/serial/generic.c

···
 	char *ch = (char *)urb->transfer_buffer;
 	int i;
 
+	if (!urb->actual_length)
+		return;
+
 	tty = tty_port_tty_get(&port->port);
 	if (!tty)
 		return;