Fix segfault while profiling multitasking

[tkf]

In 1.8-DEV (2ece1cd), the following script segfaults

using Profile

function spawnmany(n)
    if n > 2
        m = n ÷ 2
        t = Threads.@spawn spawnmany(m)
        spawnmany(m)
        wait(t)
    end
end

@time @profile spawnmany(2000000)

This seems to occur essentially because SIGUSR2 handler can observe jl_get_current_task()->ptls == NULL. This patch simply adds a null check in the handler.

I think the segfault is not unexpected since lastt->ptls = NULL and jl_set_pgcstack(&t->gcstack) doesn't happen atomically in the task switch:

julia/src/task.c

Lines 434 to 438 in 2ece1cd

	lastt->ptls = NULL;
	#ifdef MIGRATE_TASKS
	ptls->previous_task = lastt;
	#endif
	jl_set_pgcstack(&t->gcstack);

The above script still doesn't work in 1.8-DEV due to (presumably) some deadlock issue. But I think this patch can already be applied.

I can reproduce this in 1.7.0-rc2.31 (75961b2) so this needs to be backported. (Interestingly, I couldn't get the same issue in 1.7.0-rc1.63 e888fb8; So the bug may be introduced or surfaced between this window.)