ALSA: fix a stupid bad use of av_freep.

It caused a segfault when channel reordering was in use and the last chunk
of data was not silent.
Spotted and diagnosed by Carl Eugen Hoyos.

libavdevice/alsa-audio-common.c

@@ -234,7 +234,7 @@ av_cold int ff_alsa_close(AVFormatContext *s1)
 {
     AlsaData *s = s1->priv_data;
 
-    av_freep(s->reorder_buf);
+    av_freep(&s->reorder_buf);
     snd_pcm_close(s->h);
     return 0;
 }