Fix crash when a claim tx has some non-witness inputs.

The logger which decides what to refer to an on-chain claim tx was
assuming that all inputs would have a witness. While this was fine
for the one-input case, it broke the fuzzer which was connecting a
consensus-invalid transaction. Further, in the case we have multiple
inputs, some may not have a witness, which we shouldn't crash on.

This fixes 9df0250dbbad7449e3ec8f90532ce9197eb31997.