Fix crash and warning in AppRTCDemo

Don't dismiss the presented view controller if it's already being dismissed to clear a warning about dismissing from a view controller while a dismiss is in progress.

Remove the sample buffer delegate when capture is being stopped to avoid a crash when a delegate method is sent to a deallocated object.

BUG=webrtc:4734
R=jiayl@webrtc.org, tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/54669004.

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#9430}

talk/app/webrtc/objc/avfoundationvideocapturer.mm

@@ -142,6 +142,7 @@ static dispatch_queue_t kBackgroundQueue = nil;
   if (!_isRunning) {
     return;
   }
+  [_videoOutput setSampleBufferDelegate:nil queue:nullptr];
   AVCaptureSession* session = _captureSession;
   dispatch_async(kBackgroundQueue, ^{
     [session stopRunning];

talk/examples/objc/AppRTCDemo/ios/ARDVideoCallViewController.m

@@ -151,8 +151,10 @@
   self.remoteVideoTrack = nil;
   self.localVideoTrack = nil;
   [_client disconnect];
-  [self.presentingViewController dismissViewControllerAnimated:YES
-                                                    completion:nil];
+  if (![self isBeingDismissed]) {
+    [self.presentingViewController dismissViewControllerAnimated:YES
+                                                      completion:nil];
+  }
 }
 
 - (void)switchCamera {