BUG/MINOR: log: fix double-free error when error in parse_loger occurs

BUG/MINOR: log: fix double-free error when error in parse_loger occurs

Passing incorrect argments in parse_logger() which contains sample and
some values, value of logger->lb.smp_rgs will equal to smp_rgs and then,
if goto error: occurs we firstly free(smp_rgs) and after it
free(logger->lb.smp_rgs) inside free_logger() function causes double-free.
It has been reported in GH #3458

Fix it by checking the value of logger->lb.smp_rgs inside error:
statement in parse_logger(), if it equal to smp_rgs - just assign
NULL to logger->lb.smp_rgs.

This fix should be backported to versions 2.6 and upper.