Do not reset the IP to null.

Not sure why this was ever there.  I guess I had assumed that this would
always lookup the IP, but the rest of the code never assumed that.

www/add_logic.php

@@ -136,7 +136,6 @@ if (isset($_REQUEST['submit'])) {
                             if (!$row['address']) {
                                 array_push($errors, "IP address $ip is not valid in the ZTP system for {$name[$i]} at row ".($i + 1));
                             }
-                            $ip = null;
                         } catch (PDOException $e) {
                             array_push($errors, 'Failed to validate IP for row '.($i + 1).": {$e->getMessage()}");
                         }