apex in name_external may be invalid when using dual stack servers

When using dual-stack-servers and -4 or -6 is in use, apex in resolver.c:name_external may be set to a fctx->fwdname instance which has not been populated leading to an INSIST failure in dns_name_fullcompare.

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib        	0x0000000192e00e28 __pthread_kill + 8
1   libsystem_pthread.dylib       	0x0000000192e3343c pthread_kill + 292
2   libsystem_c.dylib             	0x0000000192d7b454 abort + 124
3   named                         	0x0000000104b9a80c assertion_failed + 552 (main.c:238)
4   libisc-9.19.7-dev.dylib       	0x00000001050af878 isc_assertion_failed + 56 (assertions.c:49)
5   libdns-9.19.7-dev.dylib       	0x0000000104cc8128 dns_name_fullcompare + 316 (name.c:464)
6   libdns-9.19.7-dev.dylib       	0x0000000104da1598 name_external + 160 (resolver.c:6824)
7   libdns-9.19.7-dev.dylib       	0x0000000104da07b0 rctx_answer_scan + 332 (resolver.c:8641)
8   libdns-9.19.7-dev.dylib       	0x0000000104da01e4 rctx_answer_positive + 132 (resolver.c:8514)
9   libdns-9.19.7-dev.dylib       	0x0000000104d9d56c rctx_answer + 128 (resolver.c:8407)
10  libdns-9.19.7-dev.dylib       	0x0000000104d99fc4 resquery_response + 3456 (resolver.c:7919)
11  libdns-9.19.7-dev.dylib       	0x0000000104c6eb20 udp_recv + 1200 (dispatch.c:605)
12  libisc-9.19.7-dev.dylib       	0x000000010509aed0 isc__nm_async_readcb + 408 (netmgr.c:2188)
13  libisc-9.19.7-dev.dylib       	0x0000000105099070 isc__nm_readcb + 332 (netmgr.c:2161)
14  libisc-9.19.7-dev.dylib       	0x00000001050add5c isc__nm_udp_read_cb + 884 (udp.c:620)
15  libuv.1.dylib                 	0x0000000105dfa614 uv__udp_io + 288
16  libuv.1.dylib                 	0x0000000105dfd7c0 uv__io_poll + 1744
17  libuv.1.dylib                 	0x0000000105dedd00 uv_run + 252
18  libisc-9.19.7-dev.dylib       	0x00000001050c9c34 loop_run + 460 (loop.c:266)
19  libisc-9.19.7-dev.dylib       	0x00000001050c8388 loop_thread + 44 (loop.c:293)
20  libisc-9.19.7-dev.dylib       	0x00000001050c824c isc_loopmgr_run + 428 (loop.c:473)
21  named                         	0x0000000104b9a510 main + 424 (main.c:1548)
22  libdyld.dylib                 	0x0000000192e51430 start + 4

options {
        dual-stack-servers { 2001:4860:4860::8888; 8.8.8.8; };
};