cryptonote_basic: check output type before using it

Reported by QuarksLab.

src/cryptonote_basic/cryptonote_format_utils.cpp

@@ -135,7 +135,14 @@ namespace cryptonote
         return false;
       }
       for (size_t n = 0; n < tx.rct_signatures.outPk.size(); ++n)
+      {
+        if (tx.vout[n].target.type() != typeid(txout_to_key))
+        {
+          LOG_PRINT_L1("Unsupported output type in tx " << get_transaction_hash(tx));
+          return false;
+        }
         rv.outPk[n].dest = rct::pk2rct(boost::get<txout_to_key>(tx.vout[n].target).key);
+      }
 
       if (!base_only)
       {