fix(inputs.cisco_telemetry_mdt): check subfield sizes to avoid panics

[powersj]

fixes: #12635

[srebhan]

Hey @powersj, I think you are radically drop out early while I'm not sure if in cases were there are no subfield-fields the subfield might contain a sensible value... Are you sure this does not break other (maybe valid) use cases?

[srebhan]

Thanks for fixing this issue @powersj!

[srebhan]

Assigning it back to you @powersj in case you want to wait for testing!

[telegraf-tiger]

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Artifact URLs

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

[anpedro]

tested with latest fixes, it works.