mkv: add missing vlc_input_attachment_New() error handling

modules/demux/mkv/mkv.cpp

@@ -312,6 +312,11 @@ static int Control( demux_t *p_demux, int i_query, va_list args )
                 attachment_c *a = p_sys->stored_attachments[i];
                 (*ppp_attach)[i] = vlc_input_attachment_New( a->fileName(), a->mimeType(), NULL,
                                                              a->p_data, a->size() );
+                if( !(*ppp_attach)[i] )
+                {
+                    free(*ppp_attach);
+                    return VLC_ENOMEM;
+                }
             }
             return VLC_SUCCESS;