Avoid having to use the substr/strlen-magic.
Also strlen(PATH_site) can be statically cached.

Change-Id: I0ef942e331e2039e2ece9a55dd740db2a3896e2c
Resolves: #54126
Releases: 6.2
Reviewed-on: https://review.typo3.org/25851
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Markus Klein
Tested-by: Markus Klein