llbsolver: add input validation to policy recompute #4973

Job	Run time
govulncheck	58s
prepare	4s
binaries (darwin/amd64)	1m 17s
binaries (darwin/arm64)	58s
binaries (linux/amd64)	1m 55s
binaries (linux/arm/v7)	2m 12s
binaries (linux/arm64)	1m 54s
binaries (linux/s390x)	2m 17s
binaries (linux/ppc64le)	1m 56s
binaries (linux/riscv64)	2m 1s
binaries (windows/amd64)	1m 35s
binaries (windows/arm64)	1m 40s
test / prepare	2m 24s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	8m 34s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 49s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 59s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	9m 59s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 12s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 11s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 13s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 5s
test / run (./..., 1, integration gateway)	4m 27s
test / run (./client, containerd, nydus, integration)	7m 40s
test / run (./client, oci, nydus, integration)	5m 12s
test / run (./..., nydus, 1, integration)	4m 11s
image	8m 38s
image (rootless)	8m 34s
release	7s
scout	0s
	1h 56m 2s

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