Handle the case where a child task tries to kill a parent while it is dying

Still looks pretty racy

src/rt/rust_task.cpp

@@ -254,7 +254,7 @@ rust_task::yield(size_t time_in_us) {
     LOG(this, task, "task %s @0x%" PRIxPTR " yielding for %d us",
         name, this, time_in_us);
 
-    if (killed) {
+    if (killed && !dead()) {
         if (blocked()) {
             unblock();
         }