avconv: mark attachment streams as immediately finished

There are never any packets for those streams.
Fixes an infinite loop with -attach.

avconv_opt.c

@@ -1096,6 +1096,7 @@ static OutputStream *new_attachment_stream(OptionsContext *o, AVFormatContext *o
 {
     OutputStream *ost = new_output_stream(o, oc, AVMEDIA_TYPE_ATTACHMENT);
     ost->stream_copy = 1;
+    ost->finished    = 1;
     return ost;
 }