This patch allows to avoid panic from ng_uncallout with 11-ALPHA6 and 12-CURRENT when I unplug ethernet cable with active PPTP VPN connection.
db> bt
Tracing pid 902 tid 100675 td 0xfffff800169a1000
ng_uncallout() at ng_uncallout+0x3d/frame 0xfffffe04530b3580
ng_pptpgre_disconnect() at ng_pptpgre_disconnect+0xbb/frame 0xfffff*
ng_destroy_hook() at ng_destroy_hook+0xlfe/frame 8xfffffe84538b35d8
ng_ranode() at ng_ranode+0x75/frame 0xfffffe04538b3618
ng_apply_item() at ng_apply_itea+0x4ca/frame 0xfffffeB4538b36a8
ng_snd_item() at ng_snd_itea+0x3a9/frame 0xfffffeB4538b36e0
ngc_send() at ngc_send+0x21b/frame 0xfffffe04530b3790
sosend_generic() at sosend_generic+0x436/frame 0xfffffe04538b3850
kern_sendit() at kern_sendit+0x21b/frame Bxfffffe04538b390B
sendit() at sendit+0x19f/frame 0xfffffeB4530b3950
sys_sendto() at sys_sendto+0x4d/frame 0xfffffe04530b39a0
amd64_syscall() at amd64_syscall+0x2db/frame 0xfffffe04530b3ab0
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffeB4530b3abB
- syscall (133, FreeBSD ELF64, sys_sendto), rip = 0x80253906a, rsp - 0x7fffdfffd72B, rbp - 0x7fffdfffd770