Fix some crashes in GN.

tools/gn/tokenizer.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/tokenizer.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "tools/gn/input_file.h"
 
@@ skipping 110 matching lines @@
         type = Token::FALSE_TOKEN;
     } else if (type == Token::UNCLASSIFIED_COMMENT) {
       if (AtStartOfLine(token_begin) &&
           // If it's a standalone comment, but is a continuation of a comment on
           // a previous line, then instead make it a continued suffix comment.
           (tokens_.empty() || tokens_.back().type() != Token::SUFFIX_COMMENT ||
            tokens_.back().location().line_number() + 1 !=
                location.line_number() ||
            tokens_.back().location().char_offset() != location.char_offset())) {
         type = Token::LINE_COMMENT;
-        Advance();  // The current \n.
+        if (!at_end())  // Could be EOF.
+          Advance();  // The current \n.
         // If this comment is separated from the next syntax element, then we
         // want to tag it as a block comment. This will become a standalone
         // statement at the parser level to keep this comment separate, rather
         // than attached to the subsequent statement.
         while (!at_end() && IsCurrentWhitespace()) {
           if (IsCurrentNewline()) {
             type = Token::BLOCK_COMMENT;
             break;
           }
           Advance();
@@ skipping 247 matching lines @@
     // Different types of comments.
     help = "Comments should start with # instead";
   } else if (cur_char() == '\'') {
     help = "Strings are delimited by \" characters, not apostrophes.";
   } else {
     help = "I have no idea what this is.";
   }
 
   return Err(location, "Invalid token.", help);
 }