Windows VC++ compability fix

sql/set_var.cc

@@ -853,7 +853,7 @@ bool update_sys_var_str(sys_var_str *var_str, rw_lock_t *var_mutex,
   uint new_length= (var ? var->value->str_value.length() : 0);
   if (!old_value)
     old_value= (char*) "";
-  if (!(res= my_strdup_with_length(old_value, new_length, MYF(0))))
+  if (!(res= my_strdup_with_length((byte*) old_value, new_length, MYF(0))))
     return 1;
   /*
     Replace the old value in such a way that the any thread using