Fix several recently-introduced bugs in g_output_stream_write_async()

g_output_stream_write_async() was not initializing the newly-added
members of the WriteData structure, causing various problems.

Also, g_input_stream_read_async() was now leaking its cancellable. Fix
that as well.

https://bugzilla.gnome.org/show_bug.cgi?id=674612