node_device: avoid null dereference on error

node_device: avoid null dereference on error

If we plow on after udev_device_get_syspath fails, we will hit a NULL
dereference.  Clang found one due to strdup later in udevSetParent,
but in fact we hit a NULL dereference sooner because of the use of
STREQ within virNodeDeviceFindBySysfsPath.

* src/conf/node_device_conf.h (virNodeDeviceFindBySysfsPath): Mark
path argument non-null.
* src/node_device/node_device_udev.c (udevSetParent): Avoid null
dereference.