Initialize input queue before using it.
Hack that makes Catch not run in loop in certain situations.
Make exceptions in XML reader show up in main thread.

When the XML parser failed for some reason exceptions where thrown in the
reader thread and killed the program even if you have a try-catch-clause in the
main thread.
Fix undefined behaviour in Reader.

The constructor was calling a function on the not yet fully initialized object.
And the m_childpid as used before initialization.
Rename InputThread class to osmium::io::detail::ReadThread.
Rename OutputThread class to osmium::io::detail::WriteThread.
Instantiate CheckedTask with parameters for thread object.

Instead of the thread object itself.
Remove need for open() function in InputFormat.