replmd: Make replmd_process_linked_attribute() mem dependencies clearer

replmd: Make replmd_process_linked_attribute() mem dependencies clearer

This patch should not alter functionality - it is just making memory
assumptions used in replmd_process_linked_attribute() clearer.

When adding/removing msg->elements we have to take care, as this will
invalidate things like the parsed-DN array or old ldb_message_element
pointers. This has always been the case (i.e. f6bc4c08b19f5615a49),
however, now we need to take even more care, as the msg being modified
is re-used and split across 2 different functions.

Add more code comments to highlight this. We can also free
pdn_list/old_el to prevent them being incorrectly used after realloc.
It seems appropriate to also add a sanity-check that the tmp_ctx alloc
succeeds (which all the other memory hangs off).

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Tim Beale <timbeale@samba.org>
Autobuild-Date(master): Wed Nov 21 05:31:10 CET 2018 on sn-devel-144