talloc: fixed a use after free error

(Import from SAMBA commit 6f51a1f45bf4de062cce7a562477e8140630a53d):

this is the minimal fix for the problem Rusty found. I previously
thought that the best fix would be to change tc->parent to be valid
for all pointers, but that is expensive for realloc with large numbers
of child pointers, which is much more commmon than I expected it to
be.