kvetree_set fails when assigning "self" value

[rhaas80]

kvttree_set incorrectly frees elem->hash when a kvtree's value hash is being set to its current hash in:

/* this key already exists, delete its current hash and reset it */
if (elem->hash != NULL) {
   kvtree_delete(&elem->hash);
}
elem->hash = hash_value;

Ie this:

kvtree *hash = kvtree_new();
kvtree *value = kvtree_new();
kvtree_set(hash, "key", value);

// this fails
kvtree *current = kvtree_get(hash, "key");
kvtree_set(hash, "key", current);

A fix is (as in C++ with assignment of this in operator=) to check if the pointers hash_value and elem->hash in

KVTree/src/kvtree.c

Line 152 in cf9a903

if (elem->hash != NULL) {

are different and only free the old hash if so.

[adammoody]

@rhaas80 , that's a good idea. I hadn't initially worried about supporting this use case, so there may be similar checks that need to be added to other parts of the code.