RFR: JDK-8210252: com/sun/jdi/DebuggerThreadTest.java fails with NPE

[Gary Adams]

Here's a quick fix to avoid the NPE using a getThreadGroup() which could 
be null
if the thread has terminated.

   Issue: https://bugs.openjdk.java.net/browse/JDK-8210252

diff --git a/test/jdk/com/sun/jdi/DebuggerThreadTest.java 
b/test/jdk/com/sun/jdi/DebuggerThreadTest.java
--- a/test/jdk/com/sun/jdi/DebuggerThreadTest.java
+++ b/test/jdk/com/sun/jdi/DebuggerThreadTest.java
@@ -77,7 +77,8 @@
          int gotThreads = tg.enumerate(list, true);
          for (int i = 0; i < Math.min(gotThreads, list.length); i++){
              Thread t = list[i];
- String groupName = t.getThreadGroup().getName();
+ ThreadGroup tga = t.getThreadGroup();
+ String groupName = (tga == null ? "<completed>": tga.getName());

              System.out.println("Thread [" + i + "] group = '" +
                                 groupName +
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