libudev: validate 'udev' argument to udev_enumerate_new()

https://bugs.freedesktop.org/show_bug.cgi?id=58289

src/libudev/libudev-enumerate.c

@@ -81,6 +81,8 @@ _public_ struct udev_enumerate *udev_enumerate_new(struct udev *udev)
 {
         struct udev_enumerate *udev_enumerate;
 
+        if (udev == NULL)
+                return NULL;
         udev_enumerate = calloc(1, sizeof(struct udev_enumerate));
         if (udev_enumerate == NULL)
                 return NULL;