Small fixes for compilation warnings.

src/common/WELL1024a.c

src/knot/server/xfr-handler.c

src/knot/zone/zone-dump.c

src/tests/common/events_tests.c

src/tests/common/fdset_tests.c

src/tests/libknot/libknot/dname_tests.c

src/tests/libknot/libknot/nsec3_tests.c