Fix assertion incorrectly checking a moved-from rather than the moved-to object.

This should fix #85.

Note that this _was_ covered by tests (checked manually). Presumably, because a moved-from `std::function` is in a valid but unspecified state, it might or might not be left empty, depending on the implementation.

PiperOrigin-RevId: 324919105

Author: var-const

firestore/src/android/lambda_event_listener.h

@@ -20,7 +20,7 @@ class LambdaEventListener : public EventListener<T> {
  public:
   LambdaEventListener(std::function<void(const T&, Error)> callback)
       : callback_(firebase::Move(callback)) {
-    FIREBASE_ASSERT(callback);
+    FIREBASE_ASSERT(callback_);
   }
 
   void OnEvent(const T& value, Error error) override {
@@ -36,7 +36,7 @@ class LambdaEventListener<void> : public EventListener<void> {
  public:
   LambdaEventListener(std::function<void()> callback)
       : callback_(firebase::Move(callback)) {
-    FIREBASE_ASSERT(callback);
+    FIREBASE_ASSERT(callback_);
   }
 
   void OnEvent(Error) override { callback_(); }