MDEV-18782 mysqldump --all-databases causes segmentation fault.

Do not consider rows of Information_schema.files where
LOGFILE_GROUP_NAME is NULL.

client/mysqldump.c

@@ -4298,7 +4298,8 @@ static int dump_tablespaces(char* ts_where)
                       " EXTRA"
                       " FROM INFORMATION_SCHEMA.FILES"
                       " WHERE FILE_TYPE = 'UNDO LOG'"
-                      " AND FILE_NAME IS NOT NULL",
+                      " AND FILE_NAME IS NOT NULL"
+                      " AND LOGFILE_GROUP_NAME IS NOT NULL",
                       256, 1024);
   if(ts_where)
   {
@@ -4313,7 +4314,7 @@ static int dump_tablespaces(char* ts_where)
   }
   dynstr_append_checked(&sqlbuf,
                 " GROUP BY LOGFILE_GROUP_NAME, FILE_NAME"
-                ", ENGINE"
+                ", ENGINE, TOTAL_EXTENTS, INITIAL_SIZE"
                 " ORDER BY LOGFILE_GROUP_NAME");
 
   if (mysql_query(mysql, sqlbuf.str) ||