Fix nil pointer error in knx_listener

[pheew]

Required for all PRs:

• Updated associated README.md.
  (Don't think this is necessary but happy to make any changes if required)
• Wrote appropriate unit tests.

The KNX input plugin crashes with a nil pointer dereference error when receiving subsequent messages for an ignored group address:

2021-06-28T18:19:19Z I! [inputs.KNXListener] Ignoring message {Command:Write Source:1.1.5 Destination:1/0/0 Data:[0]} for unknown GA "1/0/0",
panic: runtime error: invalid memory address or nil pointer dereference,
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x19680b8],
goroutine 11 [running]:,
github.com/influxdata/telegraf/plugins/inputs/knx_listener.(*KNXListener).listen(0xc00050a630),
	/go/src/github.com/influxdata/telegraf/plugins/inputs/knx_listener/knx_listener.go:158 +0x418,
github.com/influxdata/telegraf/plugins/inputs/knx_listener.(*KNXListener).Start.func1(0xc00050a630),
	/go/src/github.com/influxdata/telegraf/plugins/inputs/knx_listener/knx_listener.go:133 +0x48,
created by github.com/influxdata/telegraf/plugins/inputs/knx_listener.(*KNXListener).Start,
	/go/src/github.com/influxdata/telegraf/plugins/inputs/knx_listener/knx_listener.go:131 +0x497,

I narrowed it down to a faulty boolean check which makes it still try to process an ignored KNX message. The included unit test fails without the changes made.

[telegraf-tiger]

Thanks so much for the pull request!
🤝 ✒️ Just a reminder that the CLA has not yet been signed, and we'll need it before merging. Please sign the CLA when you get a chance, then post a comment here saying !signed-cla

[pheew]

!signed-cla

[srebhan]

Looks good. Thanks for tracking this down!

[srebhan]

Please fix the linter issue with the empty line at beginning of TestRegularReceives_MultipleMessages().

[telegraf-tiger]

Looks like new artifacts were built from this PR. Get them here!

Artifact URLs

• windows_amd64.zip

• windows_i386.zip

• darwin_amd64.tar.gz

• armv6hl.rpm

• freebsd_armv7.tar.gz

• linux_armhf.tar.gz

• armhf.deb

• armel.rpm

• linux_armel.tar.gz

• armel.deb

• aarch64.rpm

• linux_arm64.tar.gz

• arm64.deb

• ppc64le.rpm

• linux_ppc64le.tar.gz

• ppc64el.deb

• i386.rpm

• freebsd_i386.tar.gz

• linux_i386.tar.gz

• i386.deb

• linux_mips.tar.gz

• mips.deb

• linux_mipsel.tar.gz

• mipsel.deb

• s390x.rpm

• linux_s390x.tar.gz

• s390x.deb

• static_linux_amd64.tar.gz

• x86_64.rpm

• freebsd_amd64.tar.gz

• linux_amd64.tar.gz

• amd64.deb

[srebhan]

LGTM.