Issue 711:  Be more careful about verifying filename lengths when writing ISO9660 archives

* Don't cast size_t to int, since this can lead to overflow
  on machines where sizeof(int) < sizeof(size_t)
* Check a + b > limit by writing it as
    a > limit || b > limit || a + b > limit
  to avoid problems when a + b wraps around.
path_table_add_entry(struct path_table *pathtbl, struct isoent *ent)
{
	ent->ptnext = NULL;
	*pathtbl->last = ent;
	pathtbl->last = &(ent->ptnext);
	pathtbl->cnt ++;
}

path_table_add_entry(struct path_table *pathtbl, struct isoent *ent)
{
	ent->ptnext = NULL;
	*pathtbl->last = ent;
	pathtbl->last = &(ent->ptnext);
	pathtbl->cnt ++;
}
