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/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index 39f01d8..b5bc2fb 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -2289,7 +2289,7 @@
                                                 &dex_file,
                                                 dex_cache,
                                                 class_loader,
-                                                &class_def,
+                                                class_def,
                                                 Runtime::Current()->GetCompilerCallbacks(),
                                                 true /* allow soft failures */,
                                                 log_level_,