Fix bug by which current_disk_position was not reset from the last

Fix bug by which current_disk_position was not reset from the last
gc-like operation.  When the files were swapped for fasdump, the
system might not lseek thinking that the pointer into the file was at
the correct place while it doesn't.  Typical failure would be
"Could not read into the scan buffer" since the file was shorter than
expected.