iothread: check iothread->ctx before aio_context_unref to avoid assertion

if iothread->ctx is set to NULL, aio_context_unref triggers the assertion:
g_source_unref: assertion 'source != NULL' failed.
The patch fixes it.

Signed-off-by: Lin Ma <lma@suse.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20160926052958.10716-1-lma@suse.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
