Fixed space allocation to add 1 entry for the volume label if present

I got an exception during a flush of the root of the Fat image. It tried to add the volume label entry to the data buffer but this buffer was to small.
In the specific case where you got exactly the number of file entries the same as the capacity and a volume label, the buffer is allocated 1 entry to small because the volume label is not part of the file entries.
Added a check for it to extend the buffer size by 1 entry if volume label is available.