Fix sanitize fail of extension sublookups

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=960331
diff --git a/src/hb-sanitize.hh b/src/hb-sanitize.hh
index 5ecd2d2..5f5b4bd 100644
--- a/src/hb-sanitize.hh
+++ b/src/hb-sanitize.hh
@@ -211,6 +211,8 @@
     this->start = this->end = nullptr;
   }
 
+  unsigned get_edit_count () { return edit_count; }
+
   bool check_range (const void *base,
 		    unsigned int len) const
   {