fix: gracefully stop camera before page exit

Author: juvanhe

miniprogram/packageComponent/pages/media/camera/camera.js

@@ -220,6 +220,8 @@ Page({
         selector.select('#webgl')
           .node(this.init)
           .exec()
+      } else if (this.listener) {
+        this.listener.stop()
       }
     })
   },