libgomp: Fix segfault with posthumous orphan tasks [PR104385]

The following patch fixes crashes with posthumous orphan tasks.
When a parent task finishes, gomp_clear_parent clears the parent
pointers of its children tasks present in the parent->children_queue.
But children that are still waiting for dependencies aren't in that
queue yet, they will be added there only when the sibling they are
waiting for exits.  Unfortunately we were adding those tasks into
the queues with the original task->parent which then causes crashes
because that task is gone and freed.  The following patch fixes that
by clearing the parent field when we schedule such task for running
by adding it into the queues and we know that the sibling task which
is about to finish has NULL parent.

2022-02-08  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/104385
	* task.c (gomp_task_run_post_handle_dependers): If parent is NULL,
	clear task->parent.
	* testsuite/libgomp.c/pr104385.c: New test.