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();
         }

src/test/run-pass/lots-a-fail.rs

@@ -0,0 +1,23 @@
+// FIXME: Importing std::task doesn't work under check-fast?!
+// xfail-fast
+use std;
+import std::task;
+import std::comm;
+import std::uint;
+
+fn die() {
+    fail;
+}
+
+fn iloop() {
+    task::unsupervise();
+    let f = die;
+    task::spawn(f);
+}
+
+fn main() {
+    for each i in uint::range(0u, 100u) {
+        let f = iloop;
+        task::spawn(f);
+    }
+}