fix(init): stop previous camera if any

If a camera is requested after initilization the previous camera
instance needs to be terminated or else this can cause errors. The
camera might be already in use or the device only allows one active
camera at a time.

See #40

Author: gruhn

src/components/QrcodeReader.vue

@@ -167,6 +167,10 @@ export default {
   methods: {
 
     async init () {
+      if (this.camera !== null) {
+        this.camera.stop()
+      }
+
       this.camera = await Camera(this.constraints, this.$refs.video)
     },