audio: fix segfault when no audio layer is provided

Audio layer may not required in case of text/video messaging only.
Return a warn message and a null pointer to avoid further random
memory access.

Change-Id: I8db9ee3cd1333da76f85d2a7bbd91bf083291328
Tuleap: #220

src/preferences.cpp

@@ -407,6 +407,9 @@ AudioPreference::createAudioLayer()
     return nullptr;
 #endif
 #endif // __ANDROID__
+
+    RING_WARN("No audio layer provided");
+    return nullptr;
 }
 
 void AudioPreference::serialize(YAML::Emitter &out)