Fixing slapo-nops (ITS#8759)

Hello

Following ITS#8759, I am looking for ideas about how to fix slapo-nops.

This overlays suppress nilpotent operations that replace an attribute values by the same values. The code is in contrib/slapd-modules/nops/nops.c

The problem is that slapo-nops assumes it can free the nilpotent struct Modifications after removing them from the list, but for instande slapo-memberof uses struct Modifications allocated on the stack.

Not freeing in slapo-nops is not a fix, since unlinked struct Modifications will not be freed anywhere else, and we will get a memory leak.

Would it make sense to add a SLAP_MOD_ONSTACK flag in struct Modification's sm_flags so that we can tell the difference? Any other idea?