fast-import: do not call diff_delta() with empty buffer

We know diff_delta() returns NULL, saying "no good delta exists for
it", when fed an empty data.  Check the length of the data in the
caller to avoid such a call.

This incidentally reduces the number of attempted deltification we
see in the final statistics.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1076,7 +1076,7 @@ static int store_object(
 		return 1;
 	}
 
-	if (last && last->data.buf && last->depth < max_depth
+	if (last && last->data.len && last->data.buf && last->depth < max_depth
 		&& dat->len > the_hash_algo->rawsz) {
 
 		delta_count_attempts_by_type[type]++;