input/rewind: add two assertions

src/input/rewind_input_plugin.c

@@ -83,6 +83,9 @@ copy_attributes(struct input_stream *dest)
 	const struct input_rewind *r = dest->data;
 	const struct input_stream *src = &r->input;
 
+	assert(dest != src);
+	assert(dest->mime != src->mime);
+
 	dest->ready = src->ready;
 	dest->seekable = src->seekable;
 	dest->error = src->error;