Unsquashfs: fix write outside destination directory exploit

An issue on Github (https://github.com/plougher/squashfs-tools/issues/72)
shows how some specially crafted Squashfs filesystems containing
invalid file names (with '/' and ..) can cause Unsquashfs to write
files outside of the destination directory.

This commit fixes this exploit by checking all names for
validity.

In doing so I have also added checks for '.' and for names that
are shorter than they should be (names in the file system should
not have '\0' terminators).

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>