Continue changing STRING length from int8_t to uint8_t.

This fix segmentation faults caused by overflow when converting from string to numeric types.

STD_TEST.st
STD_RESSOURCE.c
LOCATED_VARIABLES.h
POUS.c
test
STD_CONF.c
SFC_TEST.st
test.exe
plc.st
POUS.h
VARIABLES.csv