Return nullptr instead of crashing in NetEqTestFactory

Currently the code in NetEqTestFactory will crash when something
unexpected happens. It would be better to return a nullptr instead and
let the caller decide how to proceed.

Bug: webrtc:10337
Change-Id: I3cfdffa7e6f2016eeaa5d6e80c5dd6c954ef8485
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/127894
Reviewed-by: Pablo Barrera González <barrerap@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27226}

modules/audio_coding/neteq/tools/neteq_rtpplay.cc

@@ -379,6 +379,7 @@ int main(int argc, char* argv[]) {
 
   std::unique_ptr<webrtc::test::NetEqTest> test =
       factory.InitializeTestFromFile(/*input_filename=*/argv[1], config);
+  RTC_CHECK(test) << "ERROR: Unable to run test";
   test->Run();
   return 0;
 }