Fix reference counter falling belowing zero

This commit fixes an issue where a node's reference counter
may fall below zero when more than one tree share the same node
(rc == 1) and then the both copy the node and fetch_subtract the
rc. This is a hard to produce issue, but possible.

The rc must never fall below zero except when the node is
about to be freed by the node_free function.

This commit replaces the fetch_subtract with node_free which
correctly frees the node when the rc == -1.

btree.c

@@ -417,7 +417,7 @@ static struct btree_node *btree_node_copy(struct btree *btree,
     if (btree_rc_load(&(bnode)->rc) > 0) { \
         struct btree_node *node2 = btree_node_copy(btree, (bnode)); \
         if (!node2) { code; } \
-        btree_rc_fetch_sub(&(bnode)->rc, 1); \
+        btree_node_free(btree, bnode); \
         (bnode) = node2; \
     } \
 }