serialization: validate RingCT prunable JSON type

(cherry picked from commit fe2e0dbc59626b737301e843ca31840adf87e9d4)

src/serialization/json_object.cpp

@@ -1179,6 +1179,11 @@ void fromJsonValue(const rapidjson::Value& val, rct::rctSig& sig)
   const auto prunable = val.FindMember("prunable");
   if (prunable != val.MemberEnd())
   {
+    if (!prunable->value.IsObject())
+    {
+      throw WRONG_TYPE("json object");
+    }
+
     rct::keyV pseudo_outs = std::move(sig.get_pseudo_outs());
 
     GET_FROM_JSON_OBJECT(prunable->value, sig.p.rangeSigs, range_proofs);