psbt: add method to confirm 'finalized' status of psbt

calling `wally_psbt_finalize` doesn't return a status indicator; instead
you must call `psbt_is_finalized` to check that it's eligible for
'extraction' -- extraction will fail if the psbt is not in a finalized
state.

bitcoin/psbt.c

@@ -80,6 +80,17 @@ struct wally_psbt *new_psbt(const tal_t *ctx, const struct wally_tx *wtx)
 	return tal_steal(ctx, psbt);
 }
 
+bool psbt_is_finalized(struct wally_psbt *psbt)
+{
+	for (size_t i = 0; i < psbt->num_inputs; i++) {
+		if (!psbt->inputs[i].final_script_sig &&
+				!psbt->inputs[i].final_witness)
+			return false;
+	}
+
+	return true;
+}
+
 struct wally_psbt_input *psbt_add_input(struct wally_psbt *psbt,
 					struct wally_tx_input *input,
 				       	size_t insert_at)

bitcoin/psbt.h

@@ -21,6 +21,17 @@ void psbt_destroy(struct wally_psbt *psbt);
 struct wally_psbt *new_psbt(const tal_t *ctx,
 			    const struct wally_tx *wtx);
 
+/**
+ * psbt_is_finalized - Check if tx is ready to be extracted
+ *
+ * The libwally library requires a transaction be *ready* for
+ * extraction before it will add/append all of the sigs/witnesses
+ * onto the global transaction. This check returns true if
+ * a psbt has the finalized script sig and/or witness data populated
+ * for such a call
+ */
+bool psbt_is_finalized(struct wally_psbt *psbt);
+
 struct wally_psbt_input *psbt_add_input(struct wally_psbt *psbt,
 					struct wally_tx_input *input,
 					size_t insert_at);