iOS: Fix improved BWE field trial.

webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import "WebRTC/RTCFieldTrials.h"
 
 #include <memory>
 
 #import "WebRTC/RTCLogging.h"
 
 #include "webrtc/system_wrappers/include/field_trial_default.h"
 
 static NSString * const kRTCEnableImprovedBitrateEstimateString =
    @"WebRTC-ImprovedBitrateEstimate/Enabled/";
 
 static std::unique_ptr<char[]> gFieldTrialInitString;
 
 void RTCInitFieldTrials(RTCFieldTrialOptions options) {
   NSMutableString *fieldTrialInitString = [NSMutableString string];
-  size_t len = fieldTrialInitString.length + 1;
-  gFieldTrialInitString.reset(new char[len]);
-
   if (options & RTCFieldTrialOptionsImprovedBitrateEstimate) {
     [fieldTrialInitString appendString:kRTCEnableImprovedBitrateEstimateString];
   }
-
+  size_t len = fieldTrialInitString.length + 1;
+  gFieldTrialInitString.reset(new char[len]);
   if (![fieldTrialInitString getCString:gFieldTrialInitString.get()
                               maxLength:len
                                encoding:NSUTF8StringEncoding]) {
     RTCLogError(@"Failed to convert field trial string.");
     return;
   }
   webrtc::field_trial::InitFieldTrialsFromString(gFieldTrialInitString.get());
 }