Add another flength() check before renaming bad QWK packets

To hopefully address:
!ERROR 2 (No such file or directory) renaming s:\sbbs\data\file/somenum.rep
 (-1 bytes) to s:\sbbs\data\file/somenum.rep.6777ed7c.bad

Clearly flength() is returning -1 (file doesn't exist) at this point, so
perhaps the call to fremove() just above updated a cached directory?