lnwallet: in NewChannelReservation ensure commit fees are payable
lnwallet: in NewChannelReservation ensure commit fees are payable

In this commit, we fix a bug introduced by the recent change of
lnwire.MilliSatoshi to be an unsigned integer. After this change an
integer underflow was left undetected, as a result we’ll now
momentarily cast to a signed integer in order to ensure that both sides
can pay the proper fee.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd