We use the magic value '-1' in case the file descriptor is not yet
initialized. If somebody calls osmo_stream_*_close() before this
changes, we used to crash. Let's check for this and avoid a crash.
Also, after close let's change the fd to -1 again to mark the fd
invalidity.
Change-Id: I3aa04999ab01cb7971ee2dad45dfc31ab4142868