Free sectors may appear anywhere in FAT

[sftse]

While investigating #63 we found a file that has FREE_SECTOR in the middle of the FAT.
Since the location and number of FREE_SECTOR is arbitrary, it should be possible to hit this error if FREE_SECTOR are counted as a legitimate sector that must exist in the CFB.

Don't know what I was thinking, having FREE_SECTOR in the middle of the file is also going to increase the file size and thus num_sectors.

[sftse]

Added the fix for #63. The issue is that the FAT is inconsistent with the DIFAT. The DIFAT chain in the header is much shorter, so for some reason the DIFAT consists of multiple disconnected components and does not include the DIFAT_SECTOR that is mentioned in the FAT.

The fix is sufficient for my case, since I either only need to read the CFB or if I need to write to it the CFB is created from scratch and will be well-formed.
I expanded the initial test to see whether the allocator falls over these additional sectors when writing to the file, and indeed it does. I don't have a fix ready yet.

[sftse]

Turning the check off entirely is probably overly permissive. I'll see if I can come up with something more restrictive.