lnwallet: don't enforce new reserved value in PSBT midstep
lnwallet: don't enforce new reserved value in PSBT midstep

This change avoids enforcing new reserved value when PSBT funding is not
finished yet as new inputs and outputs may still be added that could
change the outcome of the check.

This originally failed in the scenario when funding a channel from
external wallet *and depositing to on-chain wallet* was done
simultaneously in a single transaction. If such transaction confirms
then reserved UTXO is guaranteed to be available but the check didn't
take it into account.

The enforcement still occurs in the final step of PSBT funding flow, so
it is safe. It also occurs in case of non-PSBT funding.

aakselrod/lnd,aakselrod/lnd