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/primitive.h b/runtime/primitive.h
index 3818487..0ac5f40 100644
--- a/runtime/primitive.h
+++ b/runtime/primitive.h
@@ -132,7 +132,7 @@
         return "V";
       default:
         LOG(FATAL) << "Primitive char conversion on invalid type " << static_cast<int>(type);
-        return NULL;
+        return nullptr;
     }
   }