CVE-2026-43424: usb: gadget: f_tcm: Fix NULL pointer dereferences in nexus handling

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

usb: gadget: ftcm: Fix NULL pointer dereferences in nexus handling

The tpg->tpgnexus pointer in the USB Target driver is dynamically managed and tied to userspace configuration via ConfigFS. It can be NULL if the USB host sends requests before the nexus is fully established or immediately after it is dropped.

Currently, functions like botsubmitcommand() and the data transfer paths retrieve tvnexus = tpg->tpgnexus and immediately dereference tvnexus->tvnsesess without any validation. If a malicious or misconfigured USB host sends a BOT (Bulk-Only Transport) command during this race window, it triggers a NULL pointer dereference, leading to a kernel panic (local DoS).

This exposes an inconsistent API usage within the module, as peer functions like usbgsubmitcommand() and botsendbadresponse() correctly implement a NULL check for tvnexus before proceeding.

Fix this by bringing consistency to the nexus handling. Add the missing if (!tvnexus) checks to the vulnerable BOT command and request processing paths, aborting the command gracefully with an error instead of crashing the system.