tests-fuzz: avoid zero-length variable array in fuzz_packet (OSS-Fuzz fails to build)

tests-fuzz/fuzz_packet.c

-/*  Copyright (C) 2018 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2022 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -20,7 +20,7 @@
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
-	uint8_t copy[size];
+	uint8_t copy[1 + size];
 	memcpy(copy, data, size);
 
 	knot_pkt_t *pkt = knot_pkt_new(copy, size, NULL);