AcmReceiver: Eliminate AcmReceiver::decoders_

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2351183002
Cr-Commit-Position: refs/heads/master@{#14335}
diff --git a/webrtc/modules/audio_coding/neteq/decoder_database.h b/webrtc/modules/audio_coding/neteq/decoder_database.h
index 3728d1d..296d059 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/decoder_database.h
@@ -64,9 +64,8 @@
       return decoder ? decoder->SampleRateHz() : cng_decoder_->sample_rate_hz;
     }
 
-    const SdpAudioFormat& GetFormat() const {
-      RTC_DCHECK(audio_format_);
-      return *audio_format_;
+    const SdpAudioFormat* GetFormat() const {
+      return audio_format_ ? &*audio_format_ : nullptr;
     }
 
     // Returns true if |codec_type| is comfort noise.