map: fixed deletion of non-existent keys

vlib/builtin/map.v

@@ -207,8 +207,10 @@ pub fn (n mut mapnode) delete(key string, element_size int) {
 }
 
 pub fn (m mut map) delete(key string) {
-	m.root.delete(key, m.element_size)
-	m.size--
+	if m.exists(key) {
+		m.root.delete(key, m.element_size)
+		m.size--
+	}
 }
 
 fn (m map) exists(key string) bool {

vlib/builtin/map_test.v

@@ -38,6 +38,8 @@ fn test_map() {
 	assert keys[1] == 'hello'
 	m.delete('hi')
 	assert m.size == 1
+	m.delete('aloha')
+	assert m.size == 1
 
 	assert m['hi'] == 0
 	assert m.keys().len == 1