Minor cleanup in ios/web utils

ios/web/navigation/nscoder_util.mm

 // 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 <string>
 
 #include "ios/web/navigation/nscoder_util.h"
 
 namespace web {
 namespace nscoder_util {
 
 void EncodeString(NSCoder* coder, NSString* key, const std::string& string) {
   [coder encodeBytes:reinterpret_cast<const uint8_t*>(string.data())
               length:string.size()
               forKey:key];
 }
 
 std::string DecodeString(NSCoder* decoder, NSString* key) {
-  NSUInteger length;
+  NSUInteger length = 0;
   const uint8_t* bytes = [decoder decodeBytesForKey:key returnedLength:&length];
   return std::string(reinterpret_cast<const char*>(bytes), length);
 }
 
 }  // namespace nscoder_util
 }  // namespace web