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

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

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

The `tpg->tpg_nexus` 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 `bot_submit_command()` and the data

transfer paths retrieve `tv_nexus = tpg->tpg_nexus` and immediately

dereference `tv_nexus->tvn_se_sess` 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 `usbg_submit_command()` and `bot_send_bad_response()`

correctly implement a NULL check for `tv_nexus` before proceeding.

Fix this by bringing consistency to the nexus handling. Add the

missing `if (!tv_nexus)` checks to the vulnerable BOT command and

request processing paths, aborting the command gracefully with an

error instead of crashing the system.