!217 增强验证码校验的语义，更易懂

Merge pull request !217 from Gszekt/N/A

ruoyi-framework/src/main/java/com/ruoyi/framework/shiro/service/SysLoginService.java

@@ -41,7 +41,7 @@ public class SysLoginService
     public SysUser login(String username, String password)
     {
         // 验证码校验
-        if (!StringUtils.isEmpty(ServletUtils.getRequest().getAttribute(ShiroConstants.CURRENT_CAPTCHA)))
+        if (ShiroConstants.CAPTCHA_ERROR.equals(ServletUtils.getRequest().getAttribute(ShiroConstants.CURRENT_CAPTCHA)))
         {
             AsyncManager.me().execute(AsyncFactory.recordLogininfor(username, Constants.LOGIN_FAIL, MessageUtils.message("user.jcaptcha.error")));
             throw new CaptchaException();