receive-pack: do not overstep command line argument array

Previous commit added one element to the command line, without
making sure the result fits there.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -651,7 +651,7 @@ static const char *unpack(int quiet)
 
 	if (ntohl(hdr.hdr_entries) < unpack_limit) {
 		int code, i = 0;
-		const char *unpacker[4];
+		const char *unpacker[5];
 		unpacker[i++] = "unpack-objects";
 		if (quiet)
 			unpacker[i++] = "-q";