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/oat_file_assistant_test.cc b/runtime/oat_file_assistant_test.cc
index 0c942d2..3f6b2d2 100644
--- a/runtime/oat_file_assistant_test.cc
+++ b/runtime/oat_file_assistant_test.cc
@@ -118,7 +118,7 @@
   std::string GetImageDirectory() {
     if (IsHost()) {
       const char* host_dir = getenv("ANDROID_HOST_OUT");
-      CHECK(host_dir != NULL);
+      CHECK(host_dir != nullptr);
       return std::string(host_dir) + "/framework";
     } else {
       return std::string("/data/art-test");