Ensure 'modified' field of ArtClassDefinition is always initialized.

This field was not always being initialized due to the use of a
default constructor. This meant we would go down unintended code paths
in some circumstances.

Test: ./test.py --host -j40
Bug: 36602266

Change-Id: Iba3521d4a2fa752a7ba59dcbcbbd30e0c09869ff
diff --git a/runtime/openjdkjvmti/events-inl.h b/runtime/openjdkjvmti/events-inl.h
index 4f5eb0c..d88805e 100644
--- a/runtime/openjdkjvmti/events-inl.h
+++ b/runtime/openjdkjvmti/events-inl.h
@@ -126,6 +126,7 @@
                                                          unsigned char** new_class_data) const {
   static_assert(kEvent == ArtJvmtiEvent::kClassFileLoadHookRetransformable ||
                 kEvent == ArtJvmtiEvent::kClassFileLoadHookNonRetransformable, "Unsupported event");
+  DCHECK(*new_class_data == nullptr);
   jint current_len = class_data_len;
   unsigned char* current_class_data = const_cast<unsigned char*>(class_data);
   ArtJvmTiEnv* last_env = nullptr;