CVE-2025-37813: NULL Pointer Dereference

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

usb: xhci: Fix invalid pointer dereference in Etron workaround

This check is performed before prepare_transfer() and prepare_ring(), so enqueue can already point at the final link TRB of a segment. And indeed it will, some 0.4% of times this code is called.

Then enqueue + 1 is an invalid pointer. It will crash the kernel right away or load some junk which may look like a link TRB and cause the real link TRB to be replaced with a NOOP. This wouldn't end well.

Use a functionally equivalent test which doesn't dereference the pointer and always gives correct result.

Something has crashed my machine twice in recent days while playing with an Etron HC, and a control transfer stress test ran for confirmation has just crashed it again. The same test passes with this patch applied.