Fixed indexing of RR in RRSet in function calls.

src/knot/zone/semantic-check.c

src/knot/zone/zone-load.c

src/libknot/rrset.c

src/libknot/updates/changesets.c

src/libknot/updates/ddns.c

src/libknot/updates/xfr-in.c

src/libknot/zone/zone-contents.c