bug: invalid memory address or nil pointer dereference

[a11enhuang]

Issue description

Environment

• APISIX Go Plugin Runner's version: 0.0.2
• APISIX version: 2.9
• Go version: 1.16
• OS (cmd: uname -a): Linux apisix-server-6c484454b4-gvgbt 5.14.1-1.el7.elrepo.x86_64 ci: enable #1 SMP Wed Sep 1 18:34:12 EDT 2021 x86_64 x86_64 x86_64 GNU/Linux(docker部署)

给某个路由定义好go-pre-plugin一段时间后(可能是因为长时间没有请求)，go plugin会出错导致接口无法访问。异常信息:
2021/10/08 03:03:14 [warn] 56#56: *207 [lua] init.lua:681: 2021-10-08T03:03:14.440Z ERROR server/server.go:69 panic recovered: runtime error: invalid memory address or nil pointer dereference
2021/10/8 上午11:03:14 github.com/apache/apisix-go-plugin-runner/internal/server.recoverPanic
2021/10/8 上午11:03:14 /var/lib/jenkins/workspace/build-dev-apple-fuller-apisix-go-plugin-runner/apisix-go-plugin-runner/internal/server/server.go:69
2021/10/8 上午11:03:14 runtime.gopanic
2021/10/8 上午11:03:14 /root/go/src/runtime/panic.go:1038
2021/10/8 上午11:03:14 runtime.panicmem
2021/10/8 上午11:03:14 /root/go/src/runtime/panic.go:221
2021/10/8 上午11:03:14 runtime.sigpanic
2021/10/8 上午11:03:14 /root/go/src/runtime/signal_unix.go:735
2021/10/8 上午11:03:14 github.com/google/flatbuffers/go.(*Builder).Reset
2021/10/8 上午11:03:14 /root/go-path/pkg/mod/github.com/google/flatbuffers@v2.0.0+incompatible/go/builder.go:45
2021/10/8 上午11:03:14 github.com/apache/apisix-go-plugin-runner/internal/util.PutBuilder
2021/10/8 上午11:03:14 /var/lib/jenkins/workspace/build-dev-apple-fuller-apisix-go-plugin-runner/apisix-go-plugin-runner/internal/util/pool.go:37
2021/10/8 上午11:03:14 github.com/apache/apisix-go-plugin-runner/internal/server.handleConn
2021/10/8 上午11:03:14 /var/lib/jenkins/workspace/build-dev-apple-fuller-apisix-go-plugin-runner/apisix-go-plugin-runner/internal/server/server.go:122
2021/10/8 上午11:03:14 , context: ngx.timer

[envestcc]

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