Use getCurrencyCode method rather than directly accessing member.

The transient currencyCode member is null until the getter method
has been used, so we should no access the variable directly.

This fixes a problem with OfferBook filtering, which was checking
for isFiat(currencyCode); the variable was null and isFiat returned
the wrong answer.

core/src/main/java/bisq/core/offer/Offer.java

@@ -563,7 +563,7 @@ public Optional<OfferPayload> getOfferPayload() {
     }
 
     public boolean isFiatOffer() {
-        return CurrencyUtil.isFiatCurrency(currencyCode);
+        return CurrencyUtil.isFiatCurrency(getCurrencyCode());
     }
 
     public Optional<BsqSwapOfferPayload> getBsqSwapOfferPayload() {