Fix panic in `create_tx`

[ValuedMammal]

This looks like an unnecessarily eager use of .expect that will eventually cause a panic if reveal_to_target happens to return None.

bdk_wallet/wallet/src/wallet/mod.rs

Lines 1542 to 1551 in 7cdc985

	// recording changes to the change keychain
	if let (Excess::Change { .. }, Some((keychain, index))) = (excess, drain_index) {
	let (_, index_changeset) = self
	.indexed_graph
	.index
	.reveal_to_target(keychain, index)
	.expect("must not be None");
	self.stage.merge(index_changeset.into());
	self.mark_used(keychain, index);
	}

[notmandatory]

@oleonardolima can you do the review once we have a PR for this? need to get it in to the 2.0 release.

[oleonardolima]

@oleonardolima can you do the review once we have a PR for this? need to get it in to the 2.0 release.

Yes, will do.