docker/dockerfile-upstream:master panics

[crazy-max]

relates to #5342

https://github.com/moby/buildkit/actions/runs/11064197081/job/30741927311?pr=5359

#6 [auth] docker/buildkit-syft-scanner:pull token for registry-1.docker.io
#6 DONE 0.0s

#7 resolve image config for docker-image://docker.io/docker/buildkit-syft-scanner:stable-1
#7 DONE 0.4s

#8 [auth] library/golang:pull token for registry-1.docker.io
#8 DONE 0.0s

#9 [auth] tonistiigi/xx:pull token for registry-1.docker.io
#9 DONE 0.0s

#10 [linux/amd64 internal] load metadata for docker.io/library/golang:1.22-alpine3.20
#10 DONE 0.3s

#11 [linux/amd64 internal] load metadata for docker.io/tonistiigi/xx:1.4.0
#11 DONE 0.4s
error: error while Copy (stream to stdout): read unix @->/run/buildkit/buildkitd.sock: read: connection reset by peer
ERROR: failed to receive status: rpc error: code = Unavailable desc = error reading from server: EOF

Stacktrace: https://github.com/moby/buildkit/actions/runs/11064197081/job/30741927311?pr=5359#step:15:99

  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x144b2f0]
  
  goroutine 974 [running]:
  github.com/moby/buildkit/solver/llbsolver.recomputeDigests({0x1e60a58, 0xc0007c4500}, 0xc000f8a6f0, 0xc000323ad8, {0xc0007ed630, 0x47})
  	/src/solver/llbsolver/vertex.go:205 +0x90
  github.com/moby/buildkit/solver/llbsolver.recomputeDigests({0x1e60a58, 0xc0007c4500}, 0xc000f8a6f0, 0xc000323ad8, {0xc0007ed770, 0x47})
  	/src/solver/llbsolver/vertex.go:212 +0x148
  github.com/moby/buildkit/solver/llbsolver.recomputeDigests({0x1e60a58, 0xc0007c4500}, 0xc000f8a6f0, 0xc000323ad8, {0xc0007ed810, 0x47})
  	/src/solver/llbsolver/vertex.go:212 +0x148
  github.com/moby/buildkit/solver/llbsolver.recomputeDigests({0x1e60a58, 0xc0007c4500}, 0xc000f8a6f0, 0xc000323ad8, {0xc0007ed860, 0x47})
  	/src/solver/llbsolver/vertex.go:212 +0x148
  github.com/moby/buildkit/solver/llbsolver.loadLLB({0x1e60a58, 0xc0007c4500}, 0xc000729cb0, {0x1e48ec0, 0xc000065780}, 0xc000f8a6c0)
  	/src/solver/llbsolver/vertex.go:276 +0x47e
  github.com/moby/buildkit/solver/llbsolver.Load({0x1e60a58, 0xc0007c4500}, 0xc000729cb0, {0x1e48ec0, 0xc000065780}, {0xc000f8a690, 0x5, 0x5})
  	/src/solver/llbsolver/vertex.go:157 +0xb1
  github.com/moby/buildkit/solver/llbsolver.(*llbBridge).loadResult(0xc0003ed800, {0x1e60a58, 0xc0007c4500}, 0xc000729cb0, {0xc00067c7b0, 0x1, 0xc0006bee18?}, {0x0, 0x0, 0x0})
  	/src/solver/llbsolver/bridge.go:141 +0x6db
  github.com/moby/buildkit/solver/llbsolver.(*resultProxy).loadResult(0xc000118d20, {0x1e60a58?, 0xc0007c4500?})
  	/src/solver/llbsolver/bridge.go:283 +0x6b
  github.com/moby/buildkit/solver/llbsolver.(*resultProxy).Result.func2({0x1e60a58, 0xc0007c4500})
  	/src/solver/llbsolver/bridge.go:311 +0xeb
  github.com/moby/buildkit/util/flightcontrol.(*call[...]).run(0x1e6d360)
  	/src/util/flightcontrol/flightcontrol.go:122 +0x225
  sync.(*Once).doSlow(0x0?, 0xc0006befa0?)
  	/usr/local/go/src/sync/once.go:74 +0xc2
  sync.(*Once).Do(...)
  	/usr/local/go/src/sync/once.go:65
  created by github.com/moby/buildkit/util/flightcontrol.(*call[...]).wait in goroutine 997
  	/src/util/flightcontrol/flightcontrol.go:166 +0x536

[jsternberg]

Fixed by #5374.