Fix segfault in diff-delta.c when FLEX_ARRAY is 1

aka don't do pointer arithmetics on structs that have a FLEX_ARRAY member,
or you'll end up believing your array is 1 cell off its real address.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-delta.c

@@ -264,7 +264,7 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 	index->src_size = bufsize;
 	index->hash_mask = hmask;
 
-	mem = index + 1;
+	mem = index->hash;
 	packed_hash = mem;
 	mem = packed_hash + (hsize+1);
 	packed_entry = mem;