fix: Fix a bug with URL check for avatars

Fixes a bug introduced in #2947

src/Comment.php

@@ -588,6 +588,7 @@ protected function avatar_default($default, $email, $size, $host)
                 $default = $avatar_default;
             }
         }
+
         if ('mystery' == $default) {
             $default = $host . '/avatar/ad516503a11cd5ca435acc9bb6523536?s=' . $size;
             // ad516503a11cd5ca435acc9bb6523536 == md5('unknown@gravatar.com')
@@ -597,7 +598,7 @@ protected function avatar_default($default, $email, $size, $host)
             $default = '';
         } elseif ('gravatar_default' == $default) {
             $default = $host . '/avatar/?s=' . $size;
-        } elseif (empty($email) && !\strstr($default, 'https://')) {
+        } elseif (empty($email) && !\preg_match('/^https?:\/\//', $default)) {
             $default = $host . '/avatar/?d=' . $default . '&s=' . $size;
         }
         return $default;