Fix a bug where packing an extension with bad private key causes crash.

crypto/signature_creator_nss.cc

 // Copyright (c) 2012 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 "crypto/signature_creator.h"
 
 #include <cryptohi.h>
 #include <keyhi.h>
 #include <stdlib.h>
 
@@ skipping 66 matching lines @@
     return false;
   }
 
   return true;
 }
 
 bool SignatureCreator::Final(std::vector<uint8>* signature) {
   SECItem signature_item;
   SECStatus rv = SGN_End(sign_context_, &signature_item);
   if (rv != SECSuccess) {
-    NOTREACHED();
     return false;
   }
   signature->assign(signature_item.data,
                     signature_item.data + signature_item.len);
   SECITEM_FreeItem(&signature_item, PR_FALSE);
   return true;
 }
 
 SignatureCreator::SignatureCreator()
     : key_(NULL),
       sign_context_(NULL) {
   EnsureNSSInit();
 }
 
 }  // namespace crypto