libudev: replace unique list entries in place

libudev: replace unique list entries in place

udev_list_entry_add() freed the existing entry for a name before inserting
its replacement. Today that cannot lose the old entry, but only because
re-adding the key that was just removed never makes the hashmap grow, which
is an internal detail of the hashmap. Use hashmap_ensure_replace() instead:
it updates the existing bucket in place, making it explicit that replacing
an entry neither allocates nor can drop the previous value under OOM.

Follow-up for c01130824f22ec3835a35c8ab5f9aea65195a40f