usb: fix crash when failing to attach a second usb device

when failing to attach another usb device to a domain for some reason
which has one use device attached before, the libvirtd crashed.
The crash is caused by null-pointer dereference error in invoking
usbDeviceListSteal passed in NULL value usb variable.
commit 05abd1507d introduces the bug.

src/qemu/qemu_hotplug.c

@@ -1213,7 +1213,8 @@ error:
 
 cleanup:
     usbDeviceListFree(list);
-    usbDeviceListSteal(driver->activeUsbHostdevs, usb);
+    if (usb)
+        usbDeviceListSteal(driver->activeUsbHostdevs, usb);
     return -1;
 }