io: avoid double-free when closing QIOChannelBuffer

The QIOChannelBuffer's close implementation will free
the internal data buffer. It failed to reset the pointer
to NULL though, so when the object is later finalized
it will free it a second time with predictable crash.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <1461751518-12128-3-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
