[Qt] prevent amount overflow problem with payment requests

Bitcoin amounts are stored as uint64 in the protobuf messages (see
paymentrequest.proto), but CAmount is defined as int64_t. Because
of that we need to verify that single and accumulated amounts are
in a valid range and no variable overflow has happened.

- fixes #5624 (#5622)

Thanks @SergioDemianLerner for reporting that issue and also supplying us
with a possible solution.

- add static verifyAmount() function to PaymentServer and move the logging
  on error into the function
- also add a unit test to paymentservertests.cpp

src/qt/test/paymentservertests.cpp

@@ -7,7 +7,10 @@
 #include "optionsmodel.h"
 #include "paymentrequestdata.h"
 
+#include "amount.h"
 #include "random.h"
+#include "script/script.h"
+#include "script/standard.h"
 #include "util.h"
 #include "utilstrencodings.h"
 
@@ -184,6 +187,20 @@ void PaymentServerTests::paymentServerTests()
     tempFile.close();
     QCOMPARE(PaymentServer::readPaymentRequestFromFile(tempFile.fileName(), r.paymentRequest), false);
 
+    // Payment request with amount overflow (amount is set to 21000001 BTC):
+    data = DecodeBase64(paymentrequest5_cert2_BASE64);
+    byteArray = QByteArray((const char*)&data[0], data.size());
+    r.paymentRequest.parse(byteArray);
+    // Ensure the request is initialized
+    QVERIFY(r.paymentRequest.IsInitialized());
+    // Extract address and amount from the request
+    QList<std::pair<CScript, CAmount> > sendingTos = r.paymentRequest.getPayTo();
+    foreach (const PAIRTYPE(CScript, CAmount)& sendingTo, sendingTos) {
+        CTxDestination dest;
+        if (ExtractDestination(sendingTo.first, dest))
+            QCOMPARE(PaymentServer::verifyAmount(sendingTo.second), false);
+    }
+
     delete server;
 }