Merge pull request #46450 from MrHohn/fix-proxy-healthcheck-nilpointer
Automatic merge from submit-queue (batch tested with PRs 46450, 46272, 46453, 46019, 46367)
Fix potential nil pointer dereference for kube-proxy healthcheck
Found error log from https://storage.googleapis.com/kubernetes-jenkins/logs/ci-kubernetes-e2e-gce-slow/5222:
```
I0524 20:22:35.554156 7 healthcheck.go:226] Not saving endpoints for unknown healthcheck "kube-system/kubernetes-dashboard"
I0524 20:22:35.554172 7 proxier.go:923] syncProxyRules took 40.047209ms
I0524 20:22:35.554218 7 healthcheck.go:175] Healthcheck "e2e-tests-esipp-f7djn/external-local" closed: accept tcp [::]:32027: use of closed network connection
I0524 20:22:37.416133 7 logs.go:41] http: panic serving 169.254.169.254:49216: runtime error: invalid memory address or nil pointer dereference
goroutine 1623 [running]:
net/http.(*conn).serve.func1(0xc420b27220)
/usr/local/go_k8s_patched/src/net/http/server.go:1721 +0xd0
panic(0x1c07e40, 0x2b11ee0)
/usr/local/go_k8s_patched/src/runtime/panic.go:489 +0x2cf
k8s.io/kubernetes/pkg/proxy/healthcheck.hcHandler.ServeHTTP(0xc4201aea60, 0x15, 0xc4203233a0, 0xe, 0xc4203aa280, 0x2ab51a0, 0xc420ae4d20, 0xc4202e7a00)
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/proxy/healthcheck/healthcheck.go:200 +0x86
k8s.io/kubernetes/pkg/proxy/healthcheck.(*hcHandler).ServeHTTP(0xc420798390, 0x2ab51a0, 0xc420ae4d20, 0xc4202e7a00)
<autogenerated>:8 +0x87
net/http.serverHandler.ServeHTTP(0xc4208d0210, 0x2ab51a0, 0xc420ae4d20, 0xc4202e7a00)
/usr/local/go_k8s_patched/src/net/http/server.go:2568 +0x92
net/http.(*conn).serve(0xc420b27220, 0x2ab66e0, 0xc420945380)
/usr/local/go_k8s_patched/src/net/http/server.go:1825 +0x612
created by net/http.(*Server).Serve
/usr/local/go_k8s_patched/src/net/http/server.go:2668 +0x2ce
```
So seems like it's possible healthcheck server is still serving traffic after service entry is removed.
/assign @freehan
**Release note**:
```release-note
NONE
```
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