Fix integer overflow on progress dialog
When the size of the ISO image exceeds 2^32 bytes, 2^32 bytes are subtracted from the size displayed when the progress dialog finishes, which makes it look like there was an error.
When the size of the ISO image exceeds 2^32 bytes, 2^32 bytes are subtracted from the size displayed when the progress dialog finishes, which makes it look like there was an error.