Don't segfault on Windows because of a NULL value.

This commit was SVN r20682.

opal/threads/thread.c

@@ -87,7 +87,9 @@ int opal_thread_join(opal_thread_t *t, void **thr_return)
         return OPAL_ERROR;
     }
 
+    if( NULL != **thr_return ) {
         *thr_return = (void *)((intptr_t)rc);
+    }
 
     return OPAL_SUCCESS;
 }