perf timechart: Conditionally update start_time on fork

We don't need to overwrite current task start_time on fork, so update it
only if it's zero.

Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/1404835423-23098-4-git-send-email-stfomichev@yandex-team.ru


Signed-off-by: Jiri Olsa <jolsa@kernel.org>