adapter: automatically unmap on clearing

adapter: automatically unmap on clearing

When _clear gets called between _map and _unmap, buffers
will be unreffed. If the adapter was mapped, memory leaks
may occur.
While calling _clear between _map and _unmap does not seem
like such a great idea, this is possible in the audio
encoder base class, as _clear may be called in _finish_frame.
Since the audio encoder relies on flushing to keep track of
timestamps, delaying flushing till after handle_frame seems
dangerous.
So, we unmap on clear, as the next unmap will do nothing.
This makes _clear safe to call between _map and _unmap,
while avoiding leaking the mapped buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=664133