lnwallet/channel: add more info in case of crash
lnwallet/channel: add more info in case of crash

This commit adds a call to panic in case the HTLC we are looking for is
not found in the update log. It _should_ always be there, but we have
seen crashes resulting from it not being found. Since it will crash with
a nil pointer dereference below, we instead call panic giving us a bit
more information to work with.

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