Fix #252 (#253)

src/Spout/Common/Helper/GlobalFunctionsHelper.php

@@ -260,8 +260,10 @@ class GlobalFunctionsHelper
      */
     public function ob_end_clean()
     {
+        if (ob_get_length() > 0) {
             ob_end_clean();
         }
+    }
 
     /**
      * Wrapper around global function iconv()