Set vtable in class object to null after linking.

This is follow-up work of embedding imt and vtable for
faster interface/virtual call dispatching.
Once vtable becomes embedded, the original vtable is nulled.

Change-Id: I307696657d1e283654169dbecb8f7815c42bbabc
diff --git a/runtime/interpreter/interpreter_common.h b/runtime/interpreter/interpreter_common.h
index 1bcd27e..5a1d01e 100644
--- a/runtime/interpreter/interpreter_common.h
+++ b/runtime/interpreter/interpreter_common.h
@@ -140,7 +140,8 @@
     return false;
   }
   const uint32_t vtable_idx = (is_range) ? inst->VRegB_3rc() : inst->VRegB_35c();
-  ArtMethod* const method = receiver->GetClass()->GetVTable()->GetWithoutChecks(vtable_idx);
+  CHECK(receiver->GetClass()->ShouldHaveEmbeddedImtAndVTable());
+  ArtMethod* const method = receiver->GetClass()->GetEmbeddedVTableEntry(vtable_idx);
   if (UNLIKELY(method == nullptr)) {
     CHECK(self->IsExceptionPending());
     result->SetJ(0);