output/alsa: copy the PcmExport result to the ring_buffer

.. and not the input data.

Regression from commit b1c7649edb (integer underflow).

 https://bugs.musicpd.org/view.php?id=4639

src/output/plugins/AlsaOutputPlugin.cxx

@@ -1201,8 +1201,8 @@ AlsaOutput::Play(const void *chunk, size_t size)
 		if (error)
 			std::rethrow_exception(error);
 
-		size_t bytes_written = ring_buffer->push((const uint8_t *)chunk,
-							 size);
+		size_t bytes_written = ring_buffer->push((const uint8_t *)e.data,
+							 e.size);
 		if (bytes_written > 0)
 			return pcm_export->CalcSourceSize(bytes_written);