Initialize art_method_ in HGraph.

Spotted by Ivan Maidanski!

Benign as HGraph is allocated on the arena, and arenas
are always zero initialized.

test: test-art-host
Change-Id: Id8abe421e732dcf7a760f118b16b85fe1fac7c78
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 6881d8f..fb0c889 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -341,6 +341,7 @@
         cached_long_constants_(std::less<int64_t>(), arena->Adapter(kArenaAllocConstantsMap)),
         cached_double_constants_(std::less<int64_t>(), arena->Adapter(kArenaAllocConstantsMap)),
         cached_current_method_(nullptr),
+        art_method_(nullptr),
         inexact_object_rti_(ReferenceTypeInfo::CreateInvalid()),
         osr_(osr),
         cha_single_implementation_list_(arena->Adapter(kArenaAllocCHA)) {