Fix various Disk I/O issues in all modules

Fix various Disk I/O issues in all modules

* Uninitialized class members.

* Handle NULL potential after several dynamic_cast.

* Better error result handling from several system functions
  lseek(), fcntl() can report errors which need handling.

* diskd explicit NULL dereference on broken input.
  Extremely unlikely, but worth protecting against.

 Detected by Coverity Scan. Issues 740510, 740358, 740359,
  740511, 740317, 740360, 740513, 740318, 740514