Don't special-case a zero-sized compression.

zlib actually writes a header for that case, and while ignoring that
header will get us the right data, it will also end up messing up our
stream position.  So we actually want zlib to "uncompress" even an empty
object.

unpack-objects.c

@@ -55,8 +55,6 @@ static void *get_data(unsigned long size)
 	z_stream stream;
 	void *buf = xmalloc(size);
 
-	if (!size)
-		return buf;
 	memset(&stream, 0, sizeof(stream));
 
 	stream.next_out = buf;