Fixed appending invalid EDSK offsets block
Fixed appending invalid EDSK offsets block

If track content doesn't fit the ~64K EDSK limit we have multiple
attempts to degrade the data to make it fit. Each attempt caused
a new set of sector offsets to be generated for the same track,
resulting in an oversized (and invalid) offsets block being
appended to the output image.

Affected images will show a "Warning: X bytes of unused data
found at end of file" message when reading back from them, such
as using "samdisk info image.dsk".

simonowen/samdisk,simonowen/samdisk,simonowen/samdisk