ThreadSanitizer: data race netmgr.c:157 in nm_destroy

Launch named -g compiled from c5b6f215 and stop with Ctrl-C:

WARNING: ThreadSanitizer: data race (pid=17397)
  Write of size 1 at 0x00010b721539 by main thread (mutexes: write M70, write M75):
    #0 nm_destroy netmgr.c:157 (libisc.1503.dylib:x86_64+0x19c4b)
    #1 isc_nm_destroy netmgr.c:296 (libisc.1503.dylib:x86_64+0x1a0dc)
    #2 main main.c:947 (named:x86_64+0x10000b195)

  Previous read of size 1 at 0x00010b721539 by thread T1:
    #0 nm_thread netmgr.c:351 (libisc.1503.dylib:x86_64+0x190a7)

  Location is heap block of size 267681 at 0x00010b721000 allocated by main thread:
    #0 malloc <null> (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x4e65a)
    #1 default_memalloc mem.c:685 (libisc.1503.dylib:x86_64+0x17ae1)
    #2 main main.c:895 (named:x86_64+0x10000acbe)

  Mutex M70 (0x7b2c000002d8) created at:
    #0 pthread_mutex_init <null> (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2b9a3)
    #1 isc__mutex_init mutex.c:286 (libisc.1503.dylib:x86_64+0x46c4a)
    #2 main main.c:895 (named:x86_64+0x10000acbe)

  Mutex M75 (0x00010b7214c0) created at:
    #0 pthread_mutex_init <null> (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2b9a3)
    #1 isc__mutex_init mutex.c:286 (libisc.1503.dylib:x86_64+0x46c4a)
    #2 main main.c:895 (named:x86_64+0x10000acbe)

  Thread T1 (tid=4527460, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2a9cd)
    #1 isc_thread_create thread.c:75 (libisc.1503.dylib:x86_64+0x46d1c)
    #2 main main.c:895 (named:x86_64+0x10000acbe)

SUMMARY: ThreadSanitizer: data race netmgr.c:157 in nm_destroy