Fix crash when attempting to seek in a streamed unseekable stream, like

mms://a632.v3306a.c3306.e.vm.akamaistream.net/7/632/3306/v1/multith.download.akamai.com/3306/2005/COMEDIE/1144-0186-001.wmv 
patch by alexandre % alex14fr A yahoo P fr %


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@20808 b3059339-0415-0410-9bf9-f77b7e298cf2

libmpdemux/demux_asf.c

@@ -587,6 +587,8 @@ static demuxer_t* demux_open_asf(demuxer_t* demuxer)
             sh_audio->format=sh_audio->wf->wFormatTag;
         }
     }
+    if(!demuxer->stream->seek)
+        demuxer->seekable=0;
 
     return demuxer;
 }