Initialize root_ member by default

In case of a future coding error where SetRoot() doesn't get
called prior to GetRoot(), we at least assure consistent behavior
and avoid using uninitialized memory as a pointer.

Test: TreeHugger
Bug: 197828948
Change-Id: I858a54a706283fb0f81ad0c9f60d1adfa1e89eca
diff --git a/permission/parser.h b/permission/parser.h
index 0de9952..49da2a3 100644
--- a/permission/parser.h
+++ b/permission/parser.h
@@ -95,7 +95,7 @@
   void* scanner_ = nullptr;
   YY_BUFFER_STATE buffer_;
-  Expression* root_;
+  Expression* root_ = nullptr;
   std::vector<std::string> errors_;