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/space/malloc_space.h b/runtime/gc/space/malloc_space.h
index bbf1bbb..5f3a1db 100644
--- a/runtime/gc/space/malloc_space.h
+++ b/runtime/gc/space/malloc_space.h
@@ -60,7 +60,7 @@
   // Allocate num_bytes without allowing the underlying space to grow.
   virtual mirror::Object* Alloc(Thread* self, size_t num_bytes, size_t* bytes_allocated,
                                 size_t* usable_size, size_t* bytes_tl_bulk_allocated) = 0;
-  // Return the storage space required by obj. If usable_size isn't nullptr then it is set to the
+  // Return the storage space required by obj. If usable_size isn't null then it is set to the
   // amount of the storage space that may be used by obj.
   virtual size_t AllocationSize(mirror::Object* obj, size_t* usable_size) = 0;
   virtual size_t Free(Thread* self, mirror::Object* ptr)