Fix crash when AudioStreamOGGVorbis is empty

editor/plugins/editor_preview_plugins.cpp

@@ -624,6 +624,7 @@ Ref<Texture> EditorAudioStreamPreviewPlugin::generate(const RES &p_from, const S
 	uint8_t *imgw = imgdata.ptr();
 
 	Ref<AudioStreamPlayback> playback = stream->instance_playback();
+	ERR_FAIL_COND_V(playback.is_null(), Ref<Texture>());
 
 	float len_s = stream->get_length();
 	if (len_s == 0) {