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/mirror/object.cc b/runtime/mirror/object.cc
index 04d0cd8..5dac985 100644
--- a/runtime/mirror/object.cc
+++ b/runtime/mirror/object.cc
@@ -203,7 +203,7 @@
       !runtime->GetHeap()->IsObjectValidationEnabled() || !c->IsResolved()) {
     return;
   }
-  for (Class* cur = c; cur != NULL; cur = cur->GetSuperClass()) {
+  for (Class* cur = c; cur != nullptr; cur = cur->GetSuperClass()) {
     ArtField* fields = cur->GetIFields();
     for (size_t i = 0, count = cur->NumInstanceFields(); i < count; ++i) {
       StackHandleScope<1> hs(Thread::Current());