opusdec: fixed buffer unmapping bug

When the decoder received a NULL buffer, it tried to
unmap a not mapped buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=686829

ext/opus/gstopusdec.c

@@ -453,7 +453,7 @@ opus_dec_chain_parse_data (GstOpusDec * dec, GstBuffer * buffer)
     n = opus_multistream_decode (dec->state, data, size, out_data, samples, 0);
   }
   gst_buffer_unmap (outbuf, &omap);
-  if (buf)
+  if (data != NULL)
     gst_buffer_unmap (buf, &map);
 
   if (n < 0) {