Skip digi_sync_sounds loop if !Viewer

If a sound is used, it may dereference Viewer.  If Viewer is null, this
will crash.  In 630f119,
digi_sync_sounds changed to assume Viewer is valid.  This crashes during
early startup when not using the SDL_mixer backend, even though no
sounds are in use.

Reported-by: derhass <#45>
Fixes: 630f119 ("Cache Viewer in digiobj")

similar/main/digiobj.cpp

@@ -530,6 +530,8 @@ void digi_sync_sounds()
 	}
 
 	SoundQ_process();
+	if (!Viewer)
+		return;
 	const vcobjptr_t viewer{Viewer};
 	range_for (auto &s, SoundObjects)
 	{