output/oss: return early if PcmExport::Export() returns empty array

This can happen if the DoP converter doesn't get enough source samples
for one destination quad.  This isn't a critical bug, because the OSS
plugin doesn't support DoP yet, but it's good to be prepared.

src/output/plugins/OssOutputPlugin.cxx

@@ -670,12 +670,13 @@ OssOutput::Play(const void *chunk, size_t size)
 
 #ifdef AFMT_S24_PACKED
 	const auto e = pcm_export->Export({chunk, size});
+	if (e.empty())
+		return size;
+
 	chunk = e.data;
 	size = e.size;
 #endif
 
-	assert(size > 0);
-
 	while (true) {
 		ret = fd.Write(chunk, size);
 		if (ret > 0) {