Fix a bug when we can't get default address

webrtc/base/network.h

Index: webrtc/base/network.h
diff --git a/webrtc/base/network.h b/webrtc/base/network.h
index ccd205e83d807cd87619732ef3dcc2527ec268ca..8980b5d57d8ecb4fda77c064335a7d3ab881383a 100644
--- a/webrtc/base/network.h
+++ b/webrtc/base/network.h
@@ -58,7 +58,8 @@ 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;
 };