message: [Qt] Guard MN tab from possible missing TXs in masternode.conf
  In the event that masternode.conf contains invalid TXIDs, return early
  instead of trying to further process, and thus avoiding a segfault.
diff: @@ -37,7 +37,7 @@ void MNModel::updateMNList(){
  {
  LOCK2(cs_main, pwalletMain->cs_wallet);
  const CWalletTx *walletTx = pwalletMain->GetWalletTx(txHash);
  - if (walletTx->GetDepthInMainChain() > 0) {
  + if (walletTx && walletTx->GetDepthInMainChain() > 0) {
  txAccepted = true;
  }
  }
  @@ -103,7 +103,7 @@ QVariant MNModel::data(const QModelIndex &index, int role) const
  {
  LOCK2(cs_main, pwalletMain->cs_wallet);
  const CWalletTx *walletTx = pwalletMain->GetWalletTx(rec->vin.prevout.hash);
  - txAccepted = walletTx->GetDepthInMainChain() > 0;
  + txAccepted = walletTx && walletTx->GetDepthInMainChain() > 0;
  }
  return txAccepted;
  }
  