nameserver: keep fixed pointer on contents to answer from

src/knot/modules/onlinesign/onlinesign.c

src/knot/nameserver/axfr.c

src/knot/nameserver/internet.c

src/knot/nameserver/ixfr.c

src/knot/nameserver/nsec_proofs.c

src/knot/nameserver/process_query.c

src/knot/nameserver/process_query.h

src/knot/nameserver/query_module.c

src/knot/nameserver/xfr.c