[trak] Fix, and hook up

Works beautifully!  Test coming.
diff --git a/src/hb-aat-layout-common.hh b/src/hb-aat-layout-common.hh
index 78de04f..78a27a7 100644
--- a/src/hb-aat-layout-common.hh
+++ b/src/hb-aat-layout-common.hh
@@ -540,7 +540,7 @@
   inline hb_aat_apply_context_t (hb_ot_shape_plan_t *plan_,
 				 hb_font_t *font_,
 				 hb_buffer_t *buffer_,
-				 hb_blob_t *table,
+				 hb_blob_t *blob = const_cast<hb_blob_t *> (&Null(hb_blob_t)),
 				 const ankr &ankr_table_ = Null(ankr),
 				 const char *ankr_end_ = nullptr) :
 		plan (plan_), font (font_), face (font->face), buffer (buffer_),
@@ -548,7 +548,7 @@
 		ankr_table (ankr_table_), ankr_end (ankr_end_),
 		lookup_index (0), debug_depth (0)
   {
-    sanitizer.init (table);
+    sanitizer.init (blob);
     sanitizer.set_num_glyphs (face->get_num_glyphs ());
     sanitizer.start_processing ();
     sanitizer.set_max_ops (HB_SANITIZE_MAX_OPS_MAX);