Revert "Stop readahead thread if the file isn’t opened."

IsOpen() attempts to lock our QMutex; which causes a deadlock.

This reverts commit 6b9b113b3e83e4298fb4b62ed170b9d94d9b2007.
Stop readahead thread if the file isn’t opened try #2

No point trying to read from the file when there’s nothing to read