another map bug fix + fix some warnings

vlib/builtin/map.v

@@ -143,11 +143,10 @@ fn (m map) bs(query string, start, end int, out voidptr) {
 */ 
 
 fn (m mut map) preorder_keys(node &Node) { 
-	if node.is_empty {
-		return 
-	} 
+	if !node.is_empty {
 	m._keys[m.key_i] = node.key 
 	m.key_i++ 
+	} 
 	if !isnil(node.left) { 
 		m.preorder_keys(node.left) 
 	} 

vlib/builtin/map_test.v

@@ -38,8 +38,10 @@ fn test_map() {
 	assert keys[1] == 'hello' 
 	m.delete('hi') 
 	assert m.size == 1 
+
 	assert m['hi'] == 0
 	assert m.keys().len == 1 
+	assert m.keys()[0] == 'hello' 
 	//// 
 	mut users := map[string]User{} 
 	users['1'] = User{'Peter'}