[BUGFIX] Add file existence check before processing it

This patch adds a check if a file exists before processing it.
If the file does not exist, all file functions will generate a warning.

Resolves: #61300
Releases: 6.3, 6.2
Change-Id: Ie829ec0ade8357b16f8a550d5f137593900720b9
Reviewed-on: http://review.typo3.org/32637
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>