wallet: don't cache NaN coin price

if NaN coin price is cached, historial acquisition prices are not shown
correctly since the historial prices are requested after the full
history is initially shown. As such, "No data" will be shown, even
though the user required using historical pricing.

electrum/wallet.py

@@ -1129,7 +1129,8 @@ class Abstract_Wallet(AddressSynchronizer):
             return result
         if self.txi.get(txid, {}) != {}:
             result = self.average_price(txid, price_func, ccy) * txin_value/Decimal(COIN)
-            self.coin_price_cache[cache_key] = result
+            if not result.is_nan():
+                self.coin_price_cache[cache_key] = result
             return result
         else:
             fiat_value = self.get_fiat_value(txid, ccy)