Fix a bug when we can't get default address

webrtc/base/network.h

 /*
  *  Copyright 2004 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.
  */
 
@@ skipping 40 matching lines @@
 // Makes a string key for this network. Used in the network manager's maps.
 // Network objects are keyed on interface name, network prefix and the
 // length of that prefix.
 std::string MakeNetworkKey(const std::string& name, const IPAddress& prefix,
                            int prefix_length);
 
 class DefaultLocalAddressProvider {
  public:
   virtual ~DefaultLocalAddressProvider() = default;
   // The default local address is the local address used in multi-homed endpoint
-  // when the any address (0.0.0.0 or ::) is used as the local address.
+  // when the any address (0.0.0.0 or ::) is used as the local address. It's
+  // important to check the return value as a IP family may not be enabled.
   virtual bool GetDefaultLocalAddress(int family, IPAddress* ipaddr) const = 0;
 };
 
 // Generic network manager interface. It provides list of local
 // networks.
 class NetworkManager : public DefaultLocalAddressProvider {
  public:
   typedef std::vector<Network*> NetworkList;
 
   // This enum indicates whether adapter enumeration is allowed.
@@ skipping 301 matching lines @@
   bool ignored_;
   AdapterType type_;
   int preference_;
 
   friend class NetworkManager;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_NETWORK_H_