Mksquashfs reports -nan% when run with an empty directory

[Phillip Lougher]

If Mksquashfs is run with an empty directory, the output reports -nan%
as the percentage of the compressed directory table size versus the
uncompressed directory table size, e.g.

phillip@phoenix:/tmp/TEST$ mkdir empty_dir
phillip@phoenix:/tmp/TEST$ mksquashfs empty_dir image
Parallel mksquashfs: Using 4 processors
Creating 4.0 filesystem on image, block size 131072.
[|                                                               ] 0/0 100%

Exportable Squashfs 4.0 filesystem, gzip compressed, data block size 131072
        compressed data, compressed metadata, compressed fragments,
        compressed xattrs, compressed ids
        duplicates are removed
Filesystem size 0.16 Kbytes (0.00 Mbytes)
        100.00% of uncompressed filesystem size (0.16 Kbytes)
Inode table size 30 bytes (0.03 Kbytes)
        88.24% of uncompressed inode table size (34 bytes)
Directory table size 0 bytes (0.00 Kbytes)
        -nan% of uncompressed directory table size (0 bytes)
Number of duplicate files found 0
Number of inodes 1
Number of files 0
Number of fragments 0
Number of symbolic links 0
Number of device nodes 0
Number of fifo nodes 0
Number of socket nodes 0
Number of directories 1
Number of hard-links 0
Number of ids (unique uids + gids) 2
Number of uids 1
        phillip (1000)
Number of gids 1
        users (100)

This is because the uncompressed directory table size is zero bytes.