ITS#7538 fallout from ITS#7536 fix.

Allow leaf pages to have only 1 key.

libraries/liblmdb/mdb.c

@@ -6250,7 +6250,7 @@ static int
 mdb_rebalance(MDB_cursor *mc)
 {
 	MDB_node	*node;
-	int rc;
+	int rc, minkeys;
 	unsigned int ptop;
 	MDB_cursor	mn;
 
@@ -6380,13 +6380,12 @@ mdb_rebalance(MDB_cursor *mc)
 	DPRINTF("found neighbor page %zu (%u keys, %.1f%% full)",
 	    mn.mc_pg[mn.mc_top]->mp_pgno, NUMKEYS(mn.mc_pg[mn.mc_top]), (float)PAGEFILL(mc->mc_txn->mt_env, mn.mc_pg[mn.mc_top]) / 10);
 
-	/* If the neighbor page is above threshold and has at least three
-	 * keys, move one key from it. (A page must never have fewer than
-	 * 2 keys.)
-	 *
-	 * Otherwise we should try to merge them.
+	/* If the neighbor page is above threshold and has enough keys,
+	 * move one key from it. Otherwise we should try to merge them.
+	 * (A branch page must never have less than 2 keys.)
 	 */
-	if (PAGEFILL(mc->mc_txn->mt_env, mn.mc_pg[mn.mc_top]) >= FILL_THRESHOLD && NUMKEYS(mn.mc_pg[mn.mc_top]) > 2)
+	minkeys = 1 + (IS_BRANCH(mn.mc_pg[mn.mc_top]));
+	if (PAGEFILL(mc->mc_txn->mt_env, mn.mc_pg[mn.mc_top]) >= FILL_THRESHOLD && NUMKEYS(mn.mc_pg[mn.mc_top]) > minkeys)
 		return mdb_node_move(&mn, mc);
 	else {
 		if (mc->mc_ki[ptop] == 0)