Fixed array_combine for PHP 5.3

@MorrisJobke I picked your fix from https://github.com/owncloud/core/pull/8915/files#diff-70d328f9886bee8f9168251246746875R58 to fix the memory leak in PHP 5.3.

Fixes https://github.com/owncloud/core/issues/8813

Please review @Raydiation @MorrisJobke @icewind1991