Changed IsInDesiredThreadGroup to never return null threads.

Change-Id: I1907fbbd319156be0311243d0e3ba997ba862b22
diff --git a/src/debugger.cc b/src/debugger.cc
index 5a9faa6..12ea5e2 100644
--- a/src/debugger.cc
+++ b/src/debugger.cc
@@ -1506,10 +1506,6 @@
    private:
     bool IsInDesiredThreadGroup(Object* peer)
         SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-      // Do we want threads from all thread groups?
-      if (desired_thread_group_ == NULL) {
-        return true;
-      }
       // peer might be NULL if the thread is still starting up.
       if (peer == NULL) {
         // We can't tell the debugger about this thread yet.
@@ -1518,6 +1514,10 @@
         // Doing so might help us report ZOMBIE threads too.
         return false;
       }
+      // Do we want threads from all thread groups?
+      if (desired_thread_group_ == NULL) {
+        return true;
+      }
       Object* group = soa_.DecodeField(WellKnownClasses::java_lang_Thread_group)->GetObject(peer);
       return (group == desired_thread_group_);
     }