PHP7.1 piwigo Warning: session_start(): Failed to read session data:

I got this error with php 7.1 with piwigo 2.8.5 and 2.8.6

"Warning: session_start(): Failed to read session data: user (path: ) in ./piwigo/include/common.inc.php on line 149"

So session does not start, and you can't login.

To resolve this, I have added in the file: piwigo/include/functions_session.inc.php

to the function pwg_session_read($session_id), the following to check if the data is null :

if ($result)
{
  $row = pwg_db_fetch_assoc($result);

  if (is_null($row['data'])) {
    return '';
  }
  return $row['data'];
}

else
{
  return '';
}