fix #13168, segfault when scheduling an already-finished task

(cherry picked from commit 55e840b)

base/task.jl

@@ -168,7 +168,11 @@ function task_done_hook(t::Task)
                 end
             end
         end
-        wait()
+        # if a finished task accidentally gets into the queue, wait()
+        # could return. in that case just take the next task off the queue.
+        while true
+            wait()
+        end
     end
 end
 

src/task.c

@@ -368,7 +368,9 @@ JL_THREAD jl_value_t * volatile jl_task_arg_in_transit;
 extern int jl_in_gc;
 DLLEXPORT jl_value_t *jl_switchto(jl_task_t *t, jl_value_t *arg)
 {
-    if (t->state == done_sym || t->state == failed_sym) {
+    if (t->state == done_sym || t->state == failed_sym ||
+        // task started but stkbuf NULL'd => finish_task ran
+        (t->last != NULL && t->stkbuf == NULL && t != jl_current_task)) {
         if (t->exception != jl_nothing)
             jl_throw(t->exception);
         return t->result;

test/parallel.jl

@@ -517,3 +517,13 @@ let A = Any[]
     end
     @test length(A) == 1
 end
+
+# issue #13168
+function f13168(n)
+    val = 0
+    for i=1:n val+=sum(rand(n,n)^2) end
+    val
+end
+let t = schedule(@task f13168(100))
+    @test schedule(t) === t
+end