bootchart: Remove unneeded check for NULL

bootchart: Remove unneeded check for NULL

Directly before the changed line there is:

	while ((parent->next_ps && parent->pid != ps->ppid))
		parent = parent->next_ps;

which looks one element ahead of the list, hence we can rely on parent
being non null here.
If 'parent' were NULL at that while loop already, it would crash as we're
dereferencing 'parent' when checking for next_ps already.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>