1. Fix compile error with incompatible types

2. Fix function argument mismatch in README

easyflash/plugins/types/README.md

@@ -142,11 +142,11 @@ ef_set_struct("
 
 /* 获取结构体类型环境变量 */
 Student *student;
-ef_get_struct("张三学生", student, stu_get_cb);
+student = ef_get_struct("张三学生", stu_get_cb);
 
 /* 打印获取到的结构体内容 */
 printf("姓名：%s 籍贯：%s \n", student->name, student->hometown.name);
 
 /* 释放获取结构体类型环境变量过程中开辟的动态内存 */
 s2jHook.free_fn(student);
-```
+```

easyflash/plugins/types/ef_types.c

@@ -82,7 +82,7 @@ long ef_get_long(const char *key) {
         return atol(value);
     } else {
         EF_INFO("Couldn't find this ENV(%s)!\n", key);
-        return NULL;
+        return 0;
     }
 }
 
@@ -96,7 +96,7 @@ double ef_get_double(const char *key) {
         return atof(value);
     } else {
         EF_INFO("Couldn't find this ENV(%s)!\n", key);
-        return NULL;
+        return 0;
     }
 }