Fix nil pointer dereference on server shutdown

[KonishchevDmitry]

2024-12-24T16:26:05.629+0300        info        prometheus-nginxlog-exporter/main.go:101        caught term terminated. exiting
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x881f0e]
goroutine 51 [running]:
github.com/martin-helmich/prometheus-nginxlog-exporter/pkg/syslog.Listen.func1()
        /home/runner/work/prometheus-nginxlog-exporter/prometheus-nginxlog-exporter/pkg/syslog/syslog.go:83 +0x6e
main.processNamespace.func2()
        /home/runner/work/prometheus-nginxlog-exporter/prometheus-nginxlog-exporter/main.go:233 +0x3f
created by main.processNamespace
        /home/runner/work/prometheus-nginxlog-exporter/prometheus-nginxlog-exporter/main.go:230 +0x23b

[qlty-cloud-legacy]

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