8147026: Convert an assert in ClassLoaderData to a guarantee

Summary: Guarantee is to ensure classloader is a valid oop will help catch the errors at an early stage rather than crashing the JVM later on in the GC
Reviewed-by: coleenp

src/share/vm/classfile/classLoaderData.inline.hpp

@@ -40,7 +40,7 @@ inline ClassLoaderData* ClassLoaderData::class_loader_data(oop loader) {
 
 
 inline ClassLoaderData *ClassLoaderDataGraph::find_or_create(Handle loader, TRAPS) {
-  assert(loader() != NULL,"Must be a class loader");
+  guarantee(loader() != NULL && loader()->is_oop(), "Loader must be oop");
   // Gets the class loader data out of the java/lang/ClassLoader object, if non-null
   // it's already in the loader_data, so no need to add
   ClassLoaderData* loader_data= java_lang_ClassLoader::loader_data(loader());