[ticket/12510] Convert & to &amp; in build_url() when no params are stripped.

PHPBB3-12510
Merge pull request #2419 from prototech/ticket/12510

[ticket/12510] Convert & to &amp; in build_url() when no params are stripped.

* prototech/ticket/12510:
  [ticket/12510] Convert & to &amp; in build_url() when no params are stripped.