Fix the mini-stream size in the root property

[ebourg]

This is a follow up to #182, which didn't cover all possibles cases of gaps in the allocation table of the mini stream.

This PR contains a unit test generating a file with the following layout:

• Mini FAT Sector 1 is unallocated
• Mini FAT Sector 2 has 8 unallocated mini sectors at the beginning
• Mini FAT Sector 3 has 4 unallocated mini sectors in the middle
• Mini FAT Sector 4 is unallocated
• Mini FAT Sector 5 has 32 unallocated mini sectors at the end
• Mini FAT Sector 6 has 64 unallocated sectors at the beginning and 16 unallocated mini sectors at the end
• Mini FAT Sector 7 is unallocated
• Mini FAT Sector 8 is unallocated

With this layout POI 5.3 computes a mini stream size of 29696 bytes, that is the sum of the "occupied" bytes (as per BATBlock.getOccupiedSize()) for each sector : (0 + 128 + 128 + 0 + (128-32) + (128-16) + 0 + 0) x 64.

After signing the file with signtool:

• The first 2 mini sectors of the first mini FAT sector have been allocated (for the MsiDigitalSignatureEx entry)
• The empty mini FAT sectors 4, 7 and 8 were not removed
• The size field of the root entry is 48128 bytes, that's (5 x 128 + (128 - 16)) x 64

So the right method to compute the size is to find the absolute index of the last allocated mini sector, add one and multiply by 64.