logger: convert: fix variable type for negative value

[abonislawski]

This will fix scan tool reported issue:

time_precision variable can be used as -1

case 'f':
        config.time_precision = atoi(optarg);
   	if (config.time_precision < 0) {

case 'g':
   	config.time_precision = -1;

Fixes commit ff9343a

[marc-hb]

@dbaluta can you please get ready to test (future versions of) this with the GCC 13 compiler that produced the warning reported in #8778? Intel does not use this on the main branch.