encoder/vorbis: reset the Ogg stream after flush

Without the ogg_stream_reset() call, the "e_o_s" flag never gets
reset, and libogg writes EOS packets over and over.

src/encoder/vorbis_encoder.c

@@ -274,6 +274,8 @@ vorbis_encoder_flush(struct encoder *_encoder, G_GNUC_UNUSED GError **error)
 	vorbis_analysis_init(&encoder->vd, &encoder->vi);
 	vorbis_block_init(&encoder->vd, &encoder->vb);
 
+	ogg_stream_reset(&encoder->os);
+
 	encoder->flush = true;
 	return true;
 }