[ticket/11476] Remove pass-by-reference from sql_mutli_insert

The method never writes to the array passed by reference. So it can be
passed by value instead to avoid certain problems.

PHPBB3-11476
Merge remote-tracking branch 'nickvergessen/ticket/11476' into develop

* nickvergessen/ticket/11476:
  [ticket/11476] Remove pass-by-reference from sql_mutli_insert