normal swaps: do not try to get a refund if we have the preimage.

The other party's claim transaction might still be unconfirmed
at that point.

electrum/submarine_swaps.py

@@ -355,6 +355,9 @@ class SwapManager(Logger):
                 if remaining_time > 0:
                     # too early for refund
                     return
+                 if swap.preimage:
+                    # we have been paid. do not try to get refund.
+                    return
             else:
                 if swap.preimage is None:
                     swap.preimage = self.lnworker.get_preimage(swap.payment_hash)