security: Fix Validator::verifySignature bug

Refs: #1467

Change-Id: I2b69ccf15ede35c8b201c74ddf786e7b136fe421
diff --git a/src/security/validator.hpp b/src/security/validator.hpp
index 2b55782..32a5ab6 100644
--- a/src/security/validator.hpp
+++ b/src/security/validator.hpp
@@ -114,10 +114,10 @@
     if (interest.getName().size() < 2)
       return false;
 
-    Name signedName = interest.getName().getPrefix(-2);
+    const Name& name = interest.getName();
 
-    return verifySignature(signedName.wireEncode().value(),
-                           signedName.wireEncode().value_size(),
+    return verifySignature(name.wireEncode().value(),
+                           name.wireEncode().value_size() - name[-1].size(),
                            sig, publicKey);
   }