Assert the ThreadGroup is not null

Author: eregon

src/main/java/org/truffleruby/core/thread/ThreadManager.java

@@ -148,6 +148,7 @@ public DynamicObject createBootThread(String info) {
 
     public DynamicObject createThread(DynamicObject rubyClass, AllocateObjectNode allocateObjectNode) {
         final DynamicObject currentGroup = Layouts.THREAD.getThreadGroup(getCurrentThread());
+        assert currentGroup != null;
         final DynamicObject thread = allocateObjectNode.allocate(rubyClass,
                 packThreadFields(currentGroup, "<uninitialized>"));
         setFiberManager(thread);
@@ -156,6 +157,7 @@ public DynamicObject createThread(DynamicObject rubyClass, AllocateObjectNode al
 
     public DynamicObject createForeignThread() {
         final DynamicObject currentGroup = Layouts.THREAD.getThreadGroup(rootThread);
+        assert currentGroup != null;
         final DynamicObject thread = context.getCoreLibrary().getThreadFactory().newInstance(
                 packThreadFields(currentGroup, "<foreign thread>"));
         setFiberManager(thread);