Prevent crash when Kraken balance answer is null.

src/exchanges/kraken.cpp

@@ -72,6 +72,12 @@ void ExchangeHandlerKraken::analyzeAccountBalance()
 
 void ExchangeHandlerKraken::analyzeAccountBalanceJSON(json::value jvalue)
 {
+    if (jvalue.is_null())
+    {
+        lerr << "Kraken balance answer is null.";
+        return;
+    }
+
     if (jvalue["error"].size())
     {
         lerr << "Error in Kraken balance answer: " << jvalue["error"][0].as_string();