Improve safety of finalize_psbt

[notmandatory]

"In finalize_psbt, the code at lines 1910-1911 assumes a PSBT input corresponds to the inner transaction input. This only holds because it was checked 50 lines before (L1862). It'd be more careful to use get(n).ok_or(...)? to access the input around line 1910 too, in case the check 50 lines earlier was removed or updated. This is the kind of bug that could slip through review as it'd affect code outside the diff, and could realistically cause an issue as a PSBT is often externally provided: it can be reasonably anticipated to be turned into a remote crasher for some applications."