add 'isSet' to tell if a RippleAddress is non-empty.

src/cpp/ripple/RippleAddress.cpp

@@ -71,6 +71,11 @@ void RippleAddress::clear()
     vchData.clear();
 }
 
+bool RippleAddress::isSet() const
+{
+	return nVersion != VER_NONE;
+}
+
 std::string RippleAddress::humanAddressType() const
 {
 	switch (nVersion)