pkt: remove useless pointer sanitization in knot_pkt_free

src/knot/nameserver/update.c

src/knot/query/requestor.c

src/knot/server/tcp-handler.c

src/knot/server/udp-handler.c

src/knot/zone/zone.c

src/libknot/packet/pkt.c

src/libknot/packet/pkt.h

src/utils/kdig/kdig_exec.c

src/utils/knsupdate/knsupdate_params.c

tests-fuzz/fuzz_packet.c

tests/libknot/test_pkt.c

tests/modules/test_rrl.c

tests/test_process_query.c