7042673: LockSupport.getBlocker(null) crashes

Reviewed-by: chegar

src/share/classes/java/util/concurrent/locks/LockSupport.java

@@ -275,10 +275,14 @@ public class LockSupport {
      * snapshot -- the thread may have since unblocked or blocked on a
      * different blocker object.
      *
+     * @param t the thread
      * @return the blocker
+     * @throws NullPointerException if argument is null
      * @since 1.6
      */
     public static Object getBlocker(Thread t) {
+        if (t == null)
+            throw new NullPointerException();
         return unsafe.getObjectVolatile(t, parkBlockerOffset);
     }