Bug#43254: SQL_SELECT_LIMIT=0 crashes command line client

When asking what database is selected, client expected
to *always* get an answer from the server.

We now handle failure more gracefully.

See comments in ticket for a discussion of what happens,
and how things interlock.

client/mysql.cc

@@ -2625,7 +2625,7 @@ static void get_current_db()
       (res= mysql_use_result(&mysql)))
   {
     MYSQL_ROW row= mysql_fetch_row(res);
-    if (row[0])
+    if (row && row[0])
       current_db= my_strdup(row[0], MYF(MY_WME));
     mysql_free_result(res);
   }