Fix InsertDexFileInToClassLoader to handle null class loaders

Maybe used by legacy apps. This functionality is planned for
deprecation.

Bug: 27954959

Change-Id: I20a3f9de2ca743b2ba2a26d591c09624ead4127e
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index b4b7f34..5de502b 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -595,6 +595,7 @@
       REQUIRES(!Locks::classlinker_classes_lock_)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
+  // May be called with null class_loader due to legacy code. b/27954959
   void InsertDexFileInToClassLoader(mirror::Object* dex_file, mirror::ClassLoader* class_loader)
       REQUIRES(!Locks::classlinker_classes_lock_)
       SHARED_REQUIRES(Locks::mutator_lock_);