lnmsg: small speed-up: read first, check length after
lnmsg: small speed-up: read first, check length after

this saves around ~13% wall clock time in ChannelDB.load_data

pooler/electrum-ltc,spesmilo/electrum,vialectrum/vialectrum,pooler/electrum-ltc,pooler/electrum-ltc,wakiyamap/electrum-mona,spesmilo/electrum,vialectrum/vialectrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,spesmilo/electrum,vialectrum/vialectrum,pooler/electrum-ltc,wakiyamap/electrum-mona