Digital BitBox: Fix sending to self

Make sure that a pubkey is only appended to the checkpub array if it
corresponds to a change address. Signing will fail otherwise, if a
non-change pubkey is sent as part of the checkpub list to the Digital
BitBox.

electrum/plugins/digitalbitbox/digitalbitbox.py

@@ -570,6 +570,7 @@ class DigitalBitbox_KeyStore(Hardware_KeyStore):
                 assert o.type == TYPE_ADDRESS
                 info = tx.output_info.get(o.address)
                 if info is not None:
+                    if info.is_change:
                         index = info.address_index
                         changePath = self.get_derivation() + "/%d/%d" % index
                         changePubkey = self.derive_pubkey(index[0], index[1])