In ndn_encodeBinaryXmlData, return signedFieldsBeginOffset and signedFieldsEndOffset
diff --git a/ndn-cpp/encoding/binary-xml-wire-format.cpp b/ndn-cpp/encoding/binary-xml-wire-format.cpp
index 7556777..3ef731c 100644
--- a/ndn-cpp/encoding/binary-xml-wire-format.cpp
+++ b/ndn-cpp/encoding/binary-xml-wire-format.cpp
@@ -33,7 +33,9 @@
   interest.get(interestStruct);
 
   BinaryXmlEncoder encoder;
-  ndn_encodeBinaryXmlInterest(&interestStruct, &encoder);
+  ndn_Error error;
+  if ((error = ndn_encodeBinaryXmlInterest(&interestStruct, &encoder)))
+    throw std::runtime_error(ndn_getErrorString(error));
      
   return encoder.getOutput();
 }
@@ -64,7 +66,10 @@
   data.get(dataStruct);
 
   BinaryXmlEncoder encoder;
-  ndn_encodeBinaryXmlData(&dataStruct, &encoder);
+  unsigned int dummyBeginOffset, dummyEndOffset;
+  ndn_Error error;
+  if ((error = ndn_encodeBinaryXmlData(&dataStruct, &dummyBeginOffset, &dummyEndOffset, &encoder)))
+    throw std::runtime_error(ndn_getErrorString(error));
      
   return encoder.getOutput();
 }