fix: null pointer exception on destroy

Appears when component is destroyed right after being mounted.
Because `this.camera` hasn't been instanciated yet but it's
`stop()` function is already called.

Close #44

src/components/QrcodeReader.vue

@@ -172,7 +172,10 @@ export default {
   },
 
   beforeDestroy () {
-    this.camera.stop()
+    if (this.camera !== null) {
+      this.camera.stop()
+    }
+
     this.destroyed = true
   },