usb: xhci: Check for xhci->interrupters being allocated in xhci_mem_clearup()

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

usb: xhci: Check for xhci->interrupters being allocated in xhci_mem_clearup()

If xhci_mem_init() fails, it calls into xhci_mem_cleanup() to mop
up the damage. If it fails early enough, before xhci->interrupters
is allocated but after xhci->max_interrupters has been set, which
happens in most (all?) cases, things get uglier, as xhci_mem_cleanup()
unconditionally derefences xhci->interrupters. With prejudice.

Gate the interrupt freeing loop with a check on xhci->interrupters
being non-NULL.

Found while debugging a DMA allocation issue that led the XHCI driver
on this exact path.