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

webrtc/common_audio/signal_processing/include/signal_processing_library.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 325 matching lines @@
 // Input:
 //      - in_vector1       : Input vector 1
 //      - in_vector1_scale : Gain to be used for vector 1
 //      - in_vector2       : Input vector 2
 //      - in_vector2_scale : Gain to be used for vector 2
 //      - right_shifts     : Number of right bit shifts to be applied
 //      - length           : Number of elements in the input vectors
 //
 // Output:
 //      - out_vector       : Output vector
-// Return value            : 0 if OK, -1 if (in_vector1 == NULL
-//                           || in_vector2 == NULL || out_vector == NULL
+// Return value            : 0 if OK, -1 if (in_vector1 == null
+//                           || in_vector2 == null || out_vector == null
 //                           || length <= 0 || right_shift < 0).
 typedef int (*ScaleAndAddVectorsWithRound)(const int16_t* in_vector1,
                                            int16_t in_vector1_scale,
                                            const int16_t* in_vector2,
                                            int16_t in_vector2_scale,
                                            int right_shifts,
                                            int16_t* out_vector,
                                            size_t length);
 extern ScaleAndAddVectorsWithRound WebRtcSpl_ScaleAndAddVectorsWithRound;
 int WebRtcSpl_ScaleAndAddVectorsWithRoundC(const int16_t* in_vector1,
@@ skipping 1275 matching lines @@
 // This function multiply a 16-bit word by a 16-bit word, and accumulate this
 // value to a 32-bit integer.
 //
 // Input:
 //      - a    : The value of the first 16-bit word.
 //      - b    : The value of the second 16-bit word.
 //      - c    : The value of an 32-bit integer.
 //
 // Return Value: The value of a * b + c.
 //