fix(inputs.opcua_listener): Avoid segfault if subscription was not successful

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

Fix case where the client stops without finishing the subscription

023-08-09T12:39:47Z D! [inputs.opcua_listener] Opc Subscribe Stopped
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x2fb91ec]

goroutine 27 [running]:
github.com/gopcua/opcua.(*Subscription).Cancel(0x0, {0x80afc80, 0xc0024351d0})
        /home/sven/go/pkg/mod/github.com/gopcua/opcua@v0.4.0/subscription.go:86 +0x4c
github.com/influxdata/telegraf/plugins/inputs/opcua_listener.(*SubscribeClient).Stop(0xc000782340, {0x80afc80, 0xc0024351d0})
        /home/sven/Development/InfluxData/telegraf/plugins/inputs/opcua_listener/subscribe_client.go:82 +0x72
github.com/influxdata/telegraf/plugins/inputs/opcua_listener.(*OpcUaListener).Stop(0xc000330900)
        /home/sven/Development/InfluxData/telegraf/plugins/inputs/opcua_listener/opcua_listener.go:62 +0x6f
github.com/influxdata/telegraf/agent.stopServiceInputs({0xc000012170, 0x1, 0x0?})
        /home/sven/Development/InfluxData/telegraf/agent/agent.go:529 +0x66
github.com/influxdata/telegraf/agent.(*Agent).testRunInputs(0xc0007b22a8, {0x80afc10, 0xc002376690}, 0x0?, 0xc000de3aa0)
        /home/sven/Development/InfluxData/telegraf/agent/agent.go:519 +0x27f
github.com/influxdata/telegraf/agent.(*Agent).runTest.func4()
        /home/sven/Development/InfluxData/telegraf/agent/agent.go:1053 +0x6c
created by github.com/influxdata/telegraf/agent.(*Agent).runTest
        /home/sven/Development/InfluxData/telegraf/agent/agent.go:1051 +0x665

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz