REQUIRE when changing primaries list.

I removed a now non-existent primary server in a secondary zone configuration and named triggered a REQUIRE trying to cancel an outstanding request as the tid did not match.

REQUIRE(request->tid == isc_tid());

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib        	       0x199ad0764 __pthread_kill + 8
1   libsystem_pthread.dylib       	       0x199b07c28 pthread_kill + 288
2   libsystem_c.dylib             	       0x199a15ae8 abort + 180
3   named                         	       0x104bc03d0 assertion_failed + 408 (main.c:234)
4   libisc-9.19.17-dev.dylib      	       0x104d9c808 isc_assertion_failed + 56 (assertions.c:48)
5   libdns-9.19.17-dev.dylib      	       0x1053420c0 dns_request_cancel + 160 (request.c:749)
6   libdns-9.19.17-dev.dylib      	       0x1053bb6b8 dns_zone_setprimaries + 472 (zone.c:5953)
7   named                         	       0x104c0b370 named_zone_configure + 14504 (zoneconf.c:1811)
8   named                         	       0x104beaa5c configure_zone + 4160 (server.c:6821)
9   named                         	       0x104be03e4 configure_view + 1560 (server.c:4126)
10  named                         	       0x104bdb750 load_configuration + 9828 (server.c:9149)
11  named                         	       0x104bc8734 loadconfig + 60 (server.c:10277)
12  named                         	       0x104bc84b4 reload + 68 (server.c:10303)
13  named                         	       0x104bc82e0 named_server_reloadcommand + 180 (server.c:10632)
14  named                         	       0x104bb5d3c named_control_docommand + 2080 (control.c:250)
15  named                         	       0x104bba100 control_command + 88 (controlconf.c:401)
16  libisc-9.19.17-dev.dylib      	       0x104d9cd9c isc__async_cb + 448 (async.c:111)
17  libuv.1.dylib                 	       0x104f663c4 uv__async_io + 320
18  libuv.1.dylib                 	       0x104f761e0 uv__io_poll + 1748
19  libuv.1.dylib                 	       0x104f667bc uv_run + 244
20  libisc-9.19.17-dev.dylib      	       0x104dbddb0 loop_thread + 372 (loop.c:282)
21  libisc-9.19.17-dev.dylib      	       0x104dd9ff4 thread_body + 88 (thread.c:85)
22  libisc-9.19.17-dev.dylib      	       0x104dd9f6c isc_thread_main + 104 (thread.c:116)
23  libisc-9.19.17-dev.dylib      	       0x104dbdb0c isc_loopmgr_run + 472 (loop.c:454)
24  named                         	       0x104bc00f0 main + 508 (main.c:1592)
25  dyld                          	       0x1997aff28 start + 2236