This makes use of previous commit vfatlib improvements that allow
caller to stop chkdsk if locking fail, which will happen if volume is in use.
That way, ReactOS users won't be able any longer to f*** up their C:\ volume
by attempting to chkdsk -f it!
CORE-14087
The current implementation was broken and some dead code was never called; always failing before.
This fix revives this dead code!
Extra fix: avoid derefencing potential null-ptr.
And also, as bonus, comment the function so that logic can be easily understood.
CORE-14062
