wallet: fix paying invoice with "max" amount
wallet: fix paying invoice with "max" amount

fixes https://github.com/spesmilo/electrum/pull/7774

pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum