Don't crash if segments.geom is null.

Somtimes (for example https://www.sac-cas.ch/it/capanne-e-escursioni/portale-escursionistico-del-cas/albristhubel-6246/escursioni-con-le-racchette/von-matten-simmental-2785/)
segments.geom is null and the creation of the gpx xml subsequently fails.

If that is the case just ignore it and do not try to turn it into a
track segment.

Thanks to wizche for the report and fix!

README.md

@@ -18,3 +18,6 @@ Heed their warning and do not blindly follow the gps track! Use your brain and
 plan your route beforehand to be aware of dangerous parts and alternatives. 
 
 Take a look at their [safety instructions](https://www.sac-cas.ch/en/training-and-safety/safety/).
+
+## Contributors
+- wizche

background.js

@@ -89,9 +89,10 @@ function trackTitle(geoJson) {
  */
 function toGpx(geoJson) {
   let trackSegments = geoJson.segments.map((segment) => {
+    if (segment.geom == null) return "";
     return `<trkseg>
-            ${segment.geom.coordinates.map(toTrackPoint).join("")}
+             ${segment.geom.coordinates.map(toTrackPoint).join("")}
-        </trkseg>`;
+         </trkseg>`;
   }).join("");
 
   let endPoint = geoJson.end_point ? toWayPoint(geoJson.end_point) : "";

manifest.json

@@ -2,7 +2,7 @@
 
     "manifest_version": 2,
     "name": "SAC Route Portal GPX Downloader",
-    "version": "0.6",
+    "version": "0.7",
     "developer": {
         "name": "Sebastian Hugentobler",
         "url": "https://code.vanwa.ch/sebastian/sac-route-portal-gpx-fx"