Otherwise libwally pushes the psbt-key for 'witness script' onto the
serialized version and we fail the 'is this identical' check.
Relevant line from libwally, where if bytes, we push a psbt_key.
```
static void push_typed_varbuff(unsigned char **cursor, size_t *max,
uint64_t type,
const unsigned char *bytes, size_t bytes_len)
{
if (bytes) {
push_psbt_key(cursor, max, type, NULL, 0);
push_varbuff(cursor, max, bytes, bytes_len);
}
}
```
Reported-By: @grubles
Changelog-Fixed: openchannel_signed would fail on PSBT comparison of materially identical PSBTs
There's no stable ordering on unknown serialization, so linearizing
identical but mis-ordered unknown data will lead to 'wrong' results.
Instead, we just ignore any data that's in the psbt unknown struct.
There's probably also problems here with other PSBT maps. Really, this
needs a finer grained comparison function .... fuck
niftynei
Rusty Russell