fix(inputs.s7comm): Truncate string to actual length

[phagemann]

• Updated associated README.md. - Not necessary
• Wrote appropriate unit tests.- Not necessary
• Pull request title or commits are in conventional commit format

resolves #

Change string parsing to truncate string at actual length instead of the max string length.
Byte 0 reflects the max string length,
Byte 1 reflects the actual length

Behavior before this PR:
got [30 17 76 95 50 52 48 95 84 75 49 52 95 51 48 46 112 114 100 112 114 100 97 111 46 112 114 100 76 79 0 0] for field "ProgrammName" @ 24 --> L_240_TK14_30.prdprdao.prdLO (string)
now:
got [30 17 76 95 50 52 48 95 84 75 49 52 95 51 48 46 112 114 100 112 114 100 97 111 46 112 114 100 76 79 0 0] for field "ProgrammName" @ 24 --> L_240_TK14_30.prd (string)

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

👍 This pull request doesn't change the Telegraf binary size

📦 Click here to get additional PR build artifacts

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

[srebhan]

Thanks for fixing this bug @phagemann!