- The io cache flag seek_not_done was not set properly in the 
  reinit_io_cache function call and this led my_seek to be called
  desipite an invalid file handle.
- Added a test in reinit_io_cache to ensure we have a valid file 
  handle before setting seek_not_done flag.


mysys/mf_iocache.c:
  Added a test to only trigger my_seek function calls if we have
  a valid file descriptor.
mysys/my_seek.c:
  Refactored incomplete condition into an assertion. This also ensures
  that variable newpos is initialized properly.