REPORT-66916 fix:使用自定义mysql8驱动，连接失败

fine-druid/src/main/java/com/fr/third/alibaba/druid/util/MySqlUtils.java

@@ -386,14 +386,16 @@ public class MySqlUtils {
 
     public static long getLastPacketReceivedTimeMs(Connection conn) throws SQLException {
         if (!class_connectionImpl_Error) {
+            if (conn == null) {
+                return -1;
+            }
             try {
-                class_connectionImpl = Utils.loadClass("com.mysql.jdbc.MySQLConnection");
-                if (class_connectionImpl == null) {
-                    class_connectionImpl = Utils.loadClass("com.mysql.cj.MysqlConnection");
-                    if (class_connectionImpl != null) {
+                //REPORT-66916 这里之前获取到的是lib下的mysql5实现，而不是自定义mysql8驱动的实现
+                String className = conn.getClass().getName();
+                class_connectionImpl = Utils.loadClass(className);
+                if (className.equals("com.mysql.cj.MysqlConnection")) {
                     mysqlJdbcVersion6 = true;
                 }
-                }
             } catch (Throwable error) {
                 class_connectionImpl_Error = true;
             }