verifier: Pass ClassDef as a reference

Tiny refactor. ClassDef should never be null, yet we pass it as
a pointer, check it is not null and dereference everywhere.

Change-Id: Id89a1f599f1289d3cc00846306a890e06e438f88
diff --git a/oatdump/oatdump.cc b/oatdump/oatdump.cc
index 36421ff..c87a18b 100644
--- a/oatdump/oatdump.cc
+++ b/oatdump/oatdump.cc
@@ -1268,7 +1268,7 @@
       DCHECK(options_.class_loader_ != nullptr);
       return verifier::MethodVerifier::VerifyMethodAndDump(
           soa.Self(), vios, dex_method_idx, dex_file, dex_cache, *options_.class_loader_,
-          &class_def, code_item, nullptr, method_access_flags);
+          class_def, code_item, nullptr, method_access_flags);
     }
 
     return nullptr;