Assert error in VRT_Assign_Backend() Condition(vdir->refcnt > 0)

[nigoroll]

d8eea02 exposes what looks like a race between expiry of a newly created backend and resolution.

Assert error in VRT_Assign_Backend(), cache/cache_vrt_vcl.c line 346:
  Condition(vdir->refcnt > 0) not true.
version = varnish-trunk revision c79f57fc116a958631c79e1c24f57e729df17850, vrt api = 19.1
ident = Linux,6.1.0-15-amd64,x86_64,-jnone,-sdefault,-sdefault,-hcritbit,epoll
now = 13523.786472 (mono), 1722337142.480783 (real)
Backtrace:
  ip=0x5625db174ee5 sp=0x7fea20456180 <VBT_format+0x35>
  ip=0x5625db0ad0b3 sp=0x7fea204561a0 <pan_backtrace+0x33>
  ip=0x5625db0acdfa sp=0x7fea204561c0 <pan_ic+0x37a>
  ip=0x5625db174115 sp=0x7fea20456340 <VAS_Fail+0x55>
  ip=0x5625db0e2e17 sp=0x7fea20456390 <VRT_Assign_Backend+0x2b7>
  ip=0x7fea2f4111e6 sp=0x7fea204563c0 <dynamic_task_ref+0x86>
  ip=0x7fea2f412d50 sp=0x7fea204563e0 <dom_resolve+0x100>
  ip=0x5625db081835 sp=0x7fea20456410 <VRT_DirectorResolve+0x165>
  ip=0x7fea165faf62 sp=0x7fea20456440
  ip=0x5625db0e4e6c sp=0x7fea20456490 <vcl_call_method+0x61c>
  ip=0x5625db0e6c70 sp=0x7fea204565d0 <VCL_synth_method+0x1e0>