wallet2: don't store 0 amount outputs, they'll fail to be spent

It's better to just ignore them, the user does not really need
to know they're here. If the mask is wrong, they'll fail to be
used, and sweeping will fail as it tries to use it.

Reported by Josh Davis.

src/wallet/wallet2.cpp

@@ -1333,11 +1333,17 @@ void wallet2::scan_output(const cryptonote::transaction &tx, bool miner_tx, cons
   }
 
   THROW_WALLET_EXCEPTION_IF(std::find(outs.begin(), outs.end(), i) != outs.end(), error::wallet_internal_error, "Same output cannot be added twice");
-  outs.push_back(i);
   if (tx_scan_info.money_transfered == 0 && !miner_tx)
   {
     tx_scan_info.money_transfered = tools::decodeRct(tx.rct_signatures, tx_scan_info.received->derivation, i, tx_scan_info.mask, m_account.get_device());
   }
+  if (tx_scan_info.money_transfered == 0)
+  {
+    MERROR("Invalid output amount, skipping");
+    tx_scan_info.error = true;
+    return;
+  }
+  outs.push_back(i);
   THROW_WALLET_EXCEPTION_IF(tx_money_got_in_outs[tx_scan_info.received->index] >= std::numeric_limits<uint64_t>::max() - tx_scan_info.money_transfered,
       error::wallet_internal_error, "Overflow in received amounts");
   tx_money_got_in_outs[tx_scan_info.received->index] += tx_scan_info.money_transfered;