prevent buffer overflow from input string

[rd-blue]

StromPI3 V1.8 crashes with a buffer overflow when input string is too long. Then no command mode is possible anymore. This can happen e.g. when used with Home Assistant which is producing some messages during boot on serial0. When survived on Home Assistant status is messed up and looks like this:

show-status

Time: 00:03:00 Date: Tuesday 01.05.2018

StromPi-Output: mUSB

StromPi-Mode: mUSB

Raspberry Pi Shutdown: Disabled Shutdown-Timer: 12901 seconds

Powerfail Warning: Disabled

Serial-Less Mode: Disabled

Power Save Mode: Disabled

Power-Off Mode: Disabled

Powerfailure-Counter: 13619

PowerOn-Button: Disabled PowerOn-Button-Timer: 14384 seconds

FirmwareVersion: v1.8