json: fix json.decode with map alias (#17925)

vlib/json/json_alias_test.v

@@ -0,0 +1,26 @@
+import json
+
+pub struct User {
+	name   string
+	age    int
+	height f64
+}
+
+type Users = map[string]User
+
+const json_users = '{
+	"tom": { "name": "Tom", "age": 45, "height": 1.97 },
+	"martin": { "name": "Martin", "age": 40, "height": 1.8 }
+}'
+
+fn test_alias_with_map() {
+	a := json.decode(map[string]User, json_users)!
+	b := json.decode(Users, json_users)!
+
+	assert Users(a) == b
+
+	c := json.encode(a)
+	d := json.encode(b)
+
+	assert c == d
+}