gn: Don't read unitialized memory on files containing just a comment.

tools/gn/parser.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/parser.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 435 matching lines @@
     }
     left = (this->*infix)(std::move(left), token);
     if (has_error())
       return std::unique_ptr<ParseNode>();
   }
 
   return left;
 }
 
 std::unique_ptr<ParseNode> Parser::Block(Token token) {
-  // This entrypoing into ParseBlock means its part of an expression and we
+  // This entrypoint into ParseBlock means it's part of an expression and we
   // always want the result.
   return ParseBlock(token, BlockNode::RETURNS_SCOPE);
 }
 
 std::unique_ptr<ParseNode> Parser::Literal(Token token) {
   return base::WrapUnique(new LiteralNode(token));
 }
 
 std::unique_ptr<ParseNode> Parser::Name(Token token) {
   return IdentifierOrCall(std::unique_ptr<ParseNode>(), token);
@@ skipping 345 matching lines @@
 void Parser::AssignComments(ParseNode* file) {
   // Start by generating a pre- and post- order traversal of the tree so we
   // can determine what's before and after comments.
   std::vector<const ParseNode*> pre;
   std::vector<const ParseNode*> post;
   TraverseOrder(file, &pre, &post);
 
   // Assign line comments to syntax immediately following.
   int cur_comment = 0;
   for (auto* node : pre) {
+    if (node->GetRange().is_null()) {
+      CHECK_EQ(node, file) << "Only expected on top file node";
+      continue;
+    }
     const Location& start = node->GetRange().begin();
     while (cur_comment < static_cast<int>(line_comment_tokens_.size())) {
       if (start.byte() >= line_comment_tokens_[cur_comment].location().byte()) {
         const_cast<ParseNode*>(node)->comments_mutable()->append_before(
             line_comment_tokens_[cur_comment]);
         ++cur_comment;
       } else {
         break;
       }
     }
@@ skipping 36 matching lines @@
         break;
       }
     }
 
     // Suffix comments were assigned in reverse, so if there were multiple on
     // the same node, they need to be reversed.
     if ((*i)->comments() && !(*i)->comments()->suffix().empty())
       const_cast<ParseNode*>(*i)->comments_mutable()->ReverseSuffix();
   }
 }