xhci: check virt_dev is valid before dereferencing it

Check that the xhci_virt_dev structure that we dug out based
on a slot_id value from a command completion is valid before
dereferencing it.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210129130044.206855-7-mathias.nyman@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>