Hotfix: PHP 7.4 compatibility

Throw exception earlier if provided filename is a directory.
We except the filename to be mbox file.

Fixes laminas#7

Signed-off-by: Michał Bundyra <contact@webimpress.com>

src/Storage/Mbox.php

@@ -257,6 +257,10 @@ protected function openMboxFile($filename)
             $this->close();
         }
 
+        if (is_dir($filename)) {
+            throw new Exception\InvalidArgumentException('file is not a valid mbox file');
+        }
+
         ErrorHandler::start();
         $this->fh = fopen($filename, 'r');
         $error = ErrorHandler::stop();