Settings: Network: Test for empty netmask

There is an edge case where the netmask can be undefined, causing an
exception which breaks the UI. This handles that edge case.

Bug: 1091567
Change-Id: I40eb7e8b57670fd9582a28fa5723ac221cdf550c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364002
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799830}

ui/webui/resources/cr_components/chromeos/network/network_ip_config.js

@@ -49,11 +49,14 @@ const getRoutingPrefixAsNetmask = function(prefixLength) {
  */
 const getRoutingPrefixAsLength = function(netmask) {
   'use strict';
-  let prefixLength = 0;
+  if (!netmask) {
+    return chromeos.networkConfig.mojom.NO_ROUTING_PREFIX;
+  }
   const tokens = netmask.split('.');
   if (tokens.length !== 4) {
-    return -1;
+    return chromeos.networkConfig.mojom.NO_ROUTING_PREFIX;
   }
+  let prefixLength = 0;
   for (let i = 0; i < tokens.length; ++i) {
     const token = tokens[i];
     // If we already found the last mask and the current one is not