mount: don't call sd_device_get_property_value() with a NULL pointer

Otherwise bad thing would've happened is this was a hard assert:

+ systemd-mount --umount /dev/loop0
Assertion 'device' failed at src/libsystemd/sd-device/sd-device.c:2202, function sd_device_get_property_value(). Ignoring.

Author: mrc0mmand

src/mount/mount-tool.c

@@ -975,15 +975,14 @@ static int stop_mounts(
 }
 
 static int umount_by_device(sd_bus *bus, sd_device *dev) {
-        _cleanup_(sd_device_unrefp) sd_device *d = NULL;
         _cleanup_strv_free_ char **list = NULL;
         const char *v;
         int r, ret = 0;
 
         assert(bus);
         assert(dev);
 
-        if (sd_device_get_property_value(d, "SYSTEMD_MOUNT_WHERE", &v) >= 0)
+        if (sd_device_get_property_value(dev, "SYSTEMD_MOUNT_WHERE", &v) >= 0)
                 ret = stop_mounts(bus, v);
 
         r = sd_device_get_devname(dev, &v);