[BUGFIX] t3lib_div::unlink_tempfile does not always work on Windows

Problem: The filename created by t3lib_div::tempnam may contain
backslashes.

Solution: Process the file name through t3lib_div::fixWindowsFilePath

Change-Id: I47f8fe39ffb29894d78ac28141b88ea7abf6fd2c
Releases: 4.5, 4.6, 4.7, 6.0
Resolves: #38699
Reviewed-on: http://review.typo3.org/12684
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn