Replace NULL with nullptr or null in webrtc/audio/ and common_audio/.

webrtc/common_audio/signal_processing/include/real_fft.h

Index: webrtc/common_audio/signal_processing/include/real_fft.h
diff --git a/webrtc/common_audio/signal_processing/include/real_fft.h b/webrtc/common_audio/signal_processing/include/real_fft.h
index e7942f04c4d9f9b0f4b1d8ef435934c6384baef4..ed0db767aa5bd8acf2d0066c6e3e5583387bbd86 100644
--- a/webrtc/common_audio/signal_processing/include/real_fft.h
+++ b/webrtc/common_audio/signal_processing/include/real_fft.h
@@ -57,7 +57,7 @@ void WebRtcSpl_FreeRealFFT(struct RealFFT* self);
 //
 // Return Value:
 //   0  - FFT calculation is successful.
-//   -1 - Error with bad arguments (NULL pointers).
+//   -1 - Error with bad arguments (null pointers).
 int WebRtcSpl_RealForwardFFT(struct RealFFT* self,
                              const int16_t* real_data_in,
                              int16_t* complex_data_out);
@@ -85,7 +85,7 @@ int WebRtcSpl_RealForwardFFT(struct RealFFT* self,
 //   0 or a positive number - a value that the elements in the |real_data_out|
 //                            should be shifted left with in order to get
 //                            correct physical values.
-//   -1 - Error with bad arguments (NULL pointers).
+//   -1 - Error with bad arguments (null pointers).
 int WebRtcSpl_RealInverseFFT(struct RealFFT* self,
                              const int16_t* complex_data_in,
                              int16_t* real_data_out);