Avoid crash when not using audio (#60)

Author: drfiemost

src/audio/null/null.h

@@ -52,6 +52,7 @@ class Audio_Null: public AudioBase
     void reset () override {}
     bool write (uint_least32_t frames) override;
     void pause () override {}
+    void clearBuffer() override {}
 };
 
 #endif // AUDIO_NULL_H