FileSlack: possible overwrites in directory autoexpand feature

[importantchoice]

If a user supplies a directory plus a file in this directory as destinations for fileslack exploitation, the autoexpansion of directories could lead to multiple writes into the slack space of the same file. For instance:

$ fishy -d testfs_fat12.dd fileslack -w -m "meta.json" -d adir/afile.txt -d adir longfile.txt

would first write into adir/afile.txt, then expand adir to adir/afile.txt and then write again into the slack space of adir/afile.txt.

This is an issue in FAT fileslack implementation, but I'm not sure if the NTFS implementation is affected.