apps: Assign a proper dummy signature in the Producer app

Change-Id: I3ac1866b936abe0064b1833e61657330a8280f40
Refs: #2927
diff --git a/apps/ndn-producer.cpp b/apps/ndn-producer.cpp
index f381834..c61dd8f 100644
--- a/apps/ndn-producer.cpp
+++ b/apps/ndn-producer.cpp
@@ -120,7 +120,7 @@
   }
 
   signature.setInfo(signatureInfo);
-  signature.setValue(Block(&m_signature, sizeof(m_signature)));
+  signature.setValue(::ndn::nonNegativeIntegerBlock(::ndn::tlv::SignatureValue, m_signature));
 
   data->setSignature(signature);