usb: xhci: Fix a potential null pointer dereference in xhci_debugfs_create_endpoint()

In xhci_debugfs_create_slot(), kzalloc() can fail and
dev->debugfs_private will be NULL.
In xhci_debugfs_create_endpoint(), dev->debugfs_private is used without
any null-pointer check, and can cause a null pointer dereference.

To fix this bug, a null-pointer check is added in
xhci_debugfs_create_endpoint().

This bug is found by a runtime fuzzing tool named FIZZER written by us.

[subjet line change change, add potential -Mathais]
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-debugfs.c

@@ -440,6 +440,9 @@ void xhci_debugfs_create_endpoint(struct xhci_hcd *xhci,
 	struct xhci_ep_priv	*epriv;
 	struct xhci_slot_priv	*spriv = dev->debugfs_private;
 
+	if (!spriv)
+		return;
+
 	if (spriv->eps[ep_index])
 		return;