BUG/MINOR: log: fix leak in add_sample_to_logformat_list() error path

BUG/MINOR: log: fix leak in add_sample_to_logformat_list() error path

If add_sample_to_logformat_list() fails to allocate new logformat_node,
then we directly jump to error_free label to cleanup the node using
free_logformat_node() before returning an error.

However if the node failed to allocate, then the sample expression that
was allocated just before (not yet assigned) isn't released
(free_logformat_node() is a no-op when NULL is provided). Thus if expr
wasn't assigned to the node during early failure, then it must be manually
released.

This bug was introduced by 2462e5bcc ("BUG/MINOR: log: fix potential
lf->name memory leak") which wasn't marked for backports. It only
affects 3.0.