gn: Don't do an out-of-bound access if a file ends after 'else'

tools/gn/parser.h

Index: tools/gn/parser.h
diff --git a/tools/gn/parser.h b/tools/gn/parser.h
index 29580da2b4a2946341c760ba2b43e01ae7c2f4af..22acb472876b799becbf4d04ce51d150c5cbb65e 100644
--- a/tools/gn/parser.h
+++ b/tools/gn/parser.h
@@ -111,8 +111,13 @@ class Parser {
                 const char* error_message);
   Token Consume();
 
+  // Call this only if !at_end().
   const Token& cur_token() const { return tokens_[cur_]; }
 
+  const Token& cur_or_last_token() const {
+    return at_end() ? tokens_[tokens_.size() - 1] : cur_token();
+  }
+
   bool done() const { return at_end() || has_error(); }
   bool at_end() const { return cur_ >= tokens_.size(); }
   bool has_error() const { return err_->has_error(); }