[release-3.5] Fix etcd bootstrap: Etcd not validating database consistent index, closing database and panicking on nil backend.

[serathius]

Don't redeclare err and snapshot variable, fixing validation of consistent index and closing database on defer

err variable shared throughout the NewServer function and used on line
396 to defer decision whether backend should be closed when starting
the server failed.

snapshot variable is first defined 407, redeclared locally on line 496 and later
again used on line 625. Creation of local variable is a bug introduced
in #11888.

Checking be variable is nil is done to avoid panic from #17146

DEFAULT 2023-12-16T11:46:09.619866958Z panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xc1a07e] goroutine 1 [running]: go.etcd.io/etcd/etcdserver.NewServer.func2(0xc000452de0, 0xc000451ca0) go.etcd.io/etcd/etcdserver/server.go:345 +0x3e panic(0xed6ba0, 0xc0003261f0) /usr/local/go/src/runtime/panic.go:971 +0x499 go.uber.org/zap/zapcore.(*CheckedEntry).Write(0xc00029e630, 0xc007e7c000, 0x1, 0x1) /workspace/louhi_ws/go/pkg/mod/go.uber.org/zap@v1.10.0/zapcore/entry.go:229 +0x565 go.uber.org/zap.(*Logger).Panic(0xc000114c60, 0x1097425, 0x2a, 0xc007e7c000, 0x1, 0x1) /workspace/louhi_ws/go/pkg/mod/go.uber.org/zap@v1.10.0/logger.go:225 +0x85 go.etcd.io/etcd/etcdserver.NewServer(0x7ffd9ebad365, 0x11, 0x0, 0x0, 0x0, 0x0, 0xc00016fcb0, 0x1, 0x1, 0xc00016fa70, ...) go.etcd.io/etcd/etcdserver/server.go:473 +0x3ab3 go.etcd.io/etcd/embed.StartEtcd(0xc000404000, 0xc00015a580, 0x0, 0x0) go.etcd.io/etcd/embed/etcd.go:218 +0xa38 go.etcd.io/etcd/etcdmain.startEtcd(0xc000404000, 0x106c356, 0x6, 0xc000126a01, 0x2) go.etcd.io/etcd/etcdmain/etcd.go:302 +0x32 go.etcd.io/etcd/etcdmain.startEtcdOrProxyV2() go.etcd.io/etcd/etcdmain/etcd.go:144 +0x2cba go.etcd.io/etcd/etcdmain.Main() go.etcd.io/etcd/etcdmain/main.go:46 +0x37 main.main() go.etcd.io/etcd/main.go:28 +0x25

[serathius]

cc @ahrtr

[fuweid]

LGTM