dns_zone_getdnssecsignstats fails to lock the zone and attach the returned statistics.

dns_zone_getdnssecsignstats, dns_zone_getrequeststats and dns_zone_getrcvquerystats return a pointer to the stats rather than attaching the stats. Additionally the point is set while the zone is locked but is not locked when it is returned. These are reported by Coverity in CID 468720, 468728 and 468729.