Make snapshots in sync listener registration removal idempotent (#5945)

Author: wilhuff

Firestore/Example/Tests/Integration/API/FIRDatabaseTests.mm

@@ -568,6 +568,15 @@ - (void)testSnapshotsInSyncListenerFiresAfterListenersInSync {
   [self writeDocumentRef:ref data:@{@"foo" : @3}];
 }
 
+- (void)testSnapshotsInSyncRemoveIsIdempotent {
+  // This test merely verifies that calling remove multiple times doesn't
+  // explode.
+  auto listener = [self.db addSnapshotsInSyncListener:^(){
+  }];
+  [listener remove];
+  [listener remove];
+}
+
 - (void)testListenCanBeCalledMultipleTimes {
   FIRCollectionReference *coll = [self.db collectionWithPath:@"collection"];
   FIRDocumentReference *doc = [coll documentWithAutoID];

Firestore/core/src/api/snapshots_in_sync_listener_registration.cc

@@ -36,10 +36,12 @@ void SnapshotsInSyncListenerRegistration::Remove() {
   if (async_listener) {
     async_listener->Mute();
     async_listener_.reset();
-  }
 
-  client_->RemoveSnapshotsInSyncListener(async_listener);
-  client_.reset();
+    if (client_) {
+      client_->RemoveSnapshotsInSyncListener(async_listener);
+      client_.reset();
+    }
+  }
 }
 
 }  // namespace api