[BUGFIX] compileWhereClause causes Extbase errors with DBAL

t3lib_sqlparser::compileWhereClause iterates over the where subparts. It is called from
DBAL when running DBAL in native (MySQL) mode.

It does however resets its output string whenever the subpart is either FIND_IN_SET or
IFNULL. This causes the queries built by Extbase to fail.

Change-Id: Iea3d6f02cc9b8151f9fd6454f393ff6089567a03
Fixes: #25488
Releases: 4.6, 4.5
Reviewed-on: http://review.typo3.org/1458
Reviewed-by: Stefan Neufeind
Reviewed-by: Georg Schönweger
Tested-by: Georg Schönweger
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers