Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_coding/neteq/tools/rtpcat.cc

Index: webrtc/modules/audio_coding/neteq/tools/rtpcat.cc
diff --git a/webrtc/modules/audio_coding/neteq/tools/rtpcat.cc b/webrtc/modules/audio_coding/neteq/tools/rtpcat.cc
index e1f49f7896c473881c03d60b95cdb497967a3726..c31371ecf7e8bb577d4e1b592dc018fa9aedeab7 100644
--- a/webrtc/modules/audio_coding/neteq/tools/rtpcat.cc
+++ b/webrtc/modules/audio_coding/neteq/tools/rtpcat.cc
@@ -28,13 +28,13 @@ int main(int argc, char* argv[]) {
 
   std::unique_ptr<RtpFileWriter> output(
       RtpFileWriter::Create(RtpFileWriter::kRtpDump, argv[argc - 1]));
-  RTC_CHECK(output.get() != NULL) << "Cannot open output file.";
+  RTC_CHECK(output.get() != nullptr) << "Cannot open output file.";
   printf("Output RTP file: %s\n", argv[argc - 1]);
 
   for (int i = 1; i < argc - 1; i++) {
     std::unique_ptr<RtpFileReader> input(
         RtpFileReader::Create(RtpFileReader::kRtpDump, argv[i]));
-    RTC_CHECK(input.get() != NULL) << "Cannot open input file " << argv[i];
+    RTC_CHECK(input.get() != nullptr) << "Cannot open input file " << argv[i];
     printf("Input RTP file: %s\n", argv[i]);
 
     webrtc::test::RtpPacket packet;