udevadm: enumerate - update prev pointer properly

udevadm: enumerate - update prev pointer properly

When I tried to boot a system with 256 disks x 4 paths with
device-mapper, udevadm trigger (--type=devices) that was called
from start_udev ended up dumping a core due to a segmentation
fault.

In udev_enumerate_get_list_entry(), if it finds the devices that
should be delayed, it calls syspath_add().  If realloc() in
syspath_add() allocates the required memory at a different memory
address, referring prev->len afterward causes the segmentation
fault.

Signed-off-by: Kei Tokunaga <tokunaga.keiich@jp.fujitsu.com>