Copy fewer nodes on rebalance

This commit fixes a potential future memory error.

It can't be currently tested in production because the memmove
overwrites a "spare" slot that isn't used during the
rebalance operation. But without this fix, if at some point in
the future the spare slots are no longer located immediately
after the child nodes then a heap-buffer-overflow would have
happen.

btree.c

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 #include <stdbool.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -162,6 +163,7 @@ static void btree_node_shift_left(struct btree *btree, struct btree_node *node,
     if (!node->leaf) {
         if (for_merge) {
             index++;
+            num_items_to_shift--;
         }
         memmove(&node->children[index], &node->children[index+1],
             (num_items_to_shift+1)*sizeof(struct btree_node*));