perf thread_map: Free strlist on normal path in thread_map__new_by_tid_str()

slist needs to be freed in both error path and normal path in
thread_map__new_by_tid_str().

Fixes: b52956c9

 ("perf tools: Allow multiple threads or processes in record, stat, top")
Reviewed-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240206083228.172607-6-yangjihong1@huawei.com