Silence TSAN in bin/nsupdate/nsupdate.c

WARNING: ThreadSanitizer: data race 
  Read of size 8 at 0x000000000001 by main thread:
    #0 dns_rdataset_isassociated lib/dns/rdataset.c:143
    #1 msgresetnames lib/dns/message.c:479
    #2 msgreset lib/dns/message.c:564
    #3 dns_message_destroy lib/dns/message.c:837
    #4 cleanup nsupdate.c:3245
    #5 main nsupdate.c:3351

  Previous write of size 8 at 0x000000000001 by thread T1:
    #0 dns_rdataset_init lib/dns/rdataset.c:62
    #1 getquestions lib/dns/message.c:1147
    #2 dns_message_parse lib/dns/message.c:1740
    #3 dns_request_getresponse lib/dns/request.c:1298
    #4 update_completed nsupdate.c:2383
    #5 dispatch lib/isc/task.c:1157
    #6 run lib/isc/task.c:1331
    #7 <null> <null>

  Location is heap block of size 113 at 0x000000000012 allocated by thread T1:
    #0 malloc <null>
    #1 internal_memalloc lib/isc/mem.c:887
    #2 mem_get lib/isc/mem.c:792
    #3 isc___mempool_get lib/isc/mem.c:2083
    #4 isc__mempool_get lib/isc/mem.c:3088
    #5 dns_message_gettemprdataset lib/dns/message.c:2597
    #6 dns_message_setquerytsig lib/dns/message.c:2897
    #7 dns_request_getresponse lib/dns/request.c:1292
    #8 update_completed nsupdate.c:2383
    #9 dispatch lib/isc/task.c:1157
    #10 run lib/isc/task.c:1331
    #11 <null> <null>

  Thread T1 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create lib/isc/pthreads/thread.c:60
    #2 isc__taskmgr_create lib/isc/task.c:1468
    #3 isc_taskmgr_create lib/isc/task.c:2109
    #4 setup_system nsupdate.c:994
    #5 main nsupdate.c:3344

SUMMARY: ThreadSanitizer: data race lib/dns/rdataset.c:143 in dns_rdataset_isassociated