lnwallet/btcwallet/blockchain: properly handle nil spend report
lnwallet/btcwallet/blockchain: properly handle nil spend report

This commit adds an additional check in GetUtxo that
tests for the nil-ness of the spend report returned by
the neutrino backend. Previously, a nil error and
spend report could be returned if the rescan did not
find the output at or above the start height. This
was observed to have cause a nil pointer dereference
when the returning line attempted to access the output.
This case is now handled by returning a distinct error
signaling that the output was not found.

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