child-create: Fix double free of list of labels after migrate

child-create: Fix double free of list of labels after migrate

If a migrate of a child-create occurs then labels_i and labels_r are
freed, but the pointers are left set. If the task is subsequently
destroyed without being reused, then both of these will be double
freed.

Fix this by setting labels_i and labels_r to NULL in the migrate
method after freeing, similar to other fields that are freed.

Closes strongswan/strongswan#2552

Fixes: f9b895b49f49 ("child-create: Add support to handle security labels")