Null initialize dirty_image_objects_filename_

Fixes ASAN dex2oat.

Test: build boot image with ASAN dex2oat

Change-Id: Ia564d3bad178c7ea4768197fa04ae776023a9243
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index d57bf6c..e0ad649 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -596,6 +596,7 @@
       compiled_methods_zip_filename_(nullptr),
       compiled_methods_filename_(nullptr),
       passes_to_run_filename_(nullptr),
+      dirty_image_objects_filename_(nullptr),
       multi_image_(false),
       is_host_(false),
       elf_writers_(),