Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/gc/reference_queue.cc b/runtime/gc/reference_queue.cc
index 7be0704..4c93a4c 100644
--- a/runtime/gc/reference_queue.cc
+++ b/runtime/gc/reference_queue.cc
@@ -30,7 +30,7 @@
 }
 
 void ReferenceQueue::AtomicEnqueueIfNotEnqueued(Thread* self, mirror::Reference* ref) {
-  DCHECK(ref != NULL);
+  DCHECK(ref != nullptr);
   MutexLock mu(self, *lock_);
   if (!ref->IsEnqueued()) {
     EnqueuePendingReference(ref);
@@ -43,7 +43,7 @@
 }
 
 void ReferenceQueue::EnqueuePendingReference(mirror::Reference* ref) {
-  DCHECK(ref != NULL);
+  DCHECK(ref != nullptr);
   if (IsEmpty()) {
     // 1 element cyclic queue, ie: Reference ref = ..; ref.pendingNext = ref;
     list_ = ref;