With Independence Day less than three weeks away, Upper Gwynedd officials are making a statement about so-called safe fireworks that are often seen for sale at corner stands this time of year.    Tuesday night they denied a temporary use and occupancy permit for a fireworks vendor that wants to setup shop in the parking lot of the North Penn Marketplace at Sumneytown Pike and Valley Forge Road.   Township officials say they do not want to promote the sale of fireworks in any form because of the public safety issues involved.