优化 php8 兼容

src/Captcha.php

@@ -235,7 +235,9 @@ class Captcha
         // 输出图像
         imagepng($this->im);
         $content = ob_get_clean();
+        if (PHP_VERSION_ID < 80000) {
             imagedestroy($this->im);
+        }
 
         return response($content, 200, ['Content-Length' => strlen($content)])->contentType('image/png');
     }
@@ -338,8 +340,10 @@ class Captcha
         // Resample
         $bgImage = @imagecreatefromjpeg($gb);
         @imagecopyresampled($this->im, $bgImage, 0, 0, 0, 0, $this->imageW, $this->imageH, $width, $height);
+        if (PHP_VERSION_ID < 80000) {
             @imagedestroy($bgImage);
         }
+    }
 
     /* 加密验证码 */
     private function authcode($str)