CVE-2025-21689: USB: serial: quatech2: fix null-ptr-deref in qt2_process_read_urb()

In the Linux kernel, the following vulnerability has been resolved:

USB: serial: quatech2: fix null-ptr-deref in qt2processreadurb()

This patch addresses a null-ptr-deref in qt2processreadurb() due to an incorrect bounds check in the following:

if (newport > serial->numports) { deverr(&port->dev, "%s - port change to invalid port: %i\n", func, newport); break; }

The condition doesn't account for the valid range of the serial->port buffer, which is from 0 to serial->numports - 1. When newport is equal to serial->numports, the assignment of "port" in the following code is out-of-bounds and NULL:

serialpriv->currentport = newport; port = serial->port[serialpriv->currentport];

The fix checks if newport is greater than or equal to serial->numports indicating it is out-of-bounds.