[BUGFIX] Fix isAbsPath() on Windows

Paths starting with a slash are absolute on Windows, too.

Change-Id: Ie34449b8fc9f5e5a203044457baa64283412cf66
Resolves: #16798
Releases: 4.5, 4.6
Reviewed-on: http://review.typo3.org/3712
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers