Fix detection of runtime errors in threads.

I can only imagine this must have worked at some point in the past,
but it sure didn't now. More odd is that this only showed up some
of them time. Anyway, fixed now, and simple programs doing things
like `await start die 'foo'` will now not report errors as if they
were compile-time.

src/core/Backtrace.pm

@@ -315,15 +315,14 @@ my class Backtrace {
         my $bt = $!bt;
         for $bt.keys {
             my $p6sub := $bt[$_]<sub>;
-            if nqp::istype($p6sub, Sub) {
-                return True if $p6sub.name eq 'THREAD-ENTRY';
-            }
-            elsif nqp::istype($p6sub, ForeignCode) {
+            if nqp::istype($p6sub, ForeignCode) {
                 try {
                     my Mu $sub := nqp::getattr(nqp::decont($p6sub), ForeignCode, '$!do');
-                    return True if nqp::iseq_s(nqp::getcodename($sub), 'eval');
-                    return True if nqp::iseq_s(nqp::getcodename($sub), 'print_control');
-                    return False if nqp::iseq_s(nqp::getcodename($sub), 'compile');
+                    my str $name = nqp::getcodename($sub);
+                    return True if nqp::iseq_s($name, 'THREAD-ENTRY');
+                    return True if nqp::iseq_s($name, 'eval');
+                    return True if nqp::iseq_s($name, 'print_control');
+                    return False if nqp::iseq_s($name, 'compile');
                 }
             }
         }