Fix crash for badly formed ONC

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23757025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221599 0039d316-1c4b-4281-b951-d872f2087c98

chromeos/network/policy_util.cc

@@ -47,9 +47,10 @@ void RemoveFakeCredentials(
     if (value->GetAsDictionary(&nested_object)) {
       const onc::OncFieldSignature* field_signature =
           onc::GetFieldSignature(signature, field_name);
-
-      RemoveFakeCredentials(*field_signature->value_signature,
-                            nested_object);
+      if (field_signature)
+        RemoveFakeCredentials(*field_signature->value_signature, nested_object);
+      else
+        LOG(ERROR) << "ONC has unrecoginzed field: " << field_name;
       continue;
     }