avformat: add missing vlc_input_attachment_New() error handling

modules/demux/avformat/demux.c

@@ -462,6 +462,7 @@ int OpenDemux( vlc_object_t *p_this )
                         p_attachment = vlc_input_attachment_New(
                                 filename->value, "application/x-truetype-font",
                                 NULL, cc->extradata, (int)cc->extradata_size );
+                        if( p_attachment )
                             TAB_APPEND( p_sys->i_attachments, p_sys->attachments,
                                         p_attachment );
                     }