backswipe: ignore `undefined` handlers

`undefined` handlers are created when we [remove listeners] part-way
through the array. This fixes `backswipe` for all firmwares, but
leaves the door open for a potential firmware change.

[remove listeners]: 0f16231a43/src/jswrap_object.c (L1035-L1035)

apps/backswipe/ChangeLog

@@ -1,2 +1,3 @@
 0.01: New App!
 0.02: Don't fire if the app uses swipes already.
+0.03: Only count defined handlers in the handler array.

apps/backswipe/boot.js

@@ -26,7 +26,7 @@
     if (Bangle["#on"+eventType] === undefined) {
       return 0;
     } else if (Bangle["#on"+eventType] instanceof Array) {
-      return Bangle["#on"+eventType].length;
+      return Bangle["#on"+eventType].filter(x=>x).length;
     } else if (Bangle["#on"+eventType] !== undefined) {
       return 1;
     }

apps/backswipe/metadata.json

@@ -1,7 +1,7 @@
 { "id": "backswipe",
   "name": "Back Swipe",
   "shortName":"BackSwipe",
-  "version":"0.02",
+  "version":"0.03",
   "description": "Service that allows you to use an app's back button using left to right swipe gesture",
   "icon": "app.png",
   "tags": "back,gesture,swipe",