title: required condition is false: IsFormatSampleRateAndChannelCountValid(format)
body: <p>iOS speech kit，the SFSpeechRecognizer and AVAudioEngine work together to speech recognition, crash sometime when the mic is used by other progress.</p>
  
  <pre><code>        self.audioEngine = [[AVAudioEngine alloc] init];
          AVAudioInputNode *inputNode = self.audioEngine.inputNode;
          AVAudioFormat *nativeAudioFormat = [inputNode outputFormatForBus:0];
          __weak typeof(self)weakSelf = self;
          [inputNode installTapOnBus:0 bufferSize:1024 format:nativeAudioFormat block:^(AVAudioPCMBuffer * _Nonnull buffer, AVAudioTime * _Nonnull when) {
              [weakSelf.recognitionRequest appendAudioPCMBuffer:buffer];
          }];
          [self.audioEngine prepare];
          [self.audioEngine startAndReturnError:&amp;error];
  </code></pre>
  
  <p>Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: 'required condition is false: IsFormatSampleRateAndChannelCountValid(format)'</p>