Rawvid: rewind to the beginning of the stream when an error occur

That fixes the y4m with unknown chroma that avformat can read

modules/demux/rawvid.c

@@ -383,6 +383,7 @@ static int Open( vlc_object_t * p_this )
     return VLC_SUCCESS;
 
 error:
+    stream_Seek( p_demux->s, 0 ); // Workaround, but y4m uses stream_ReadLines
     free( p_sys );
     return VLC_EGENERIC;
 }