[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/paymentserver.h

@@ -95,6 +95,8 @@ public:
     static bool verifyNetwork(const payments::PaymentDetails& requestDetails);
     // Verify if the payment request is expired
     static bool verifyExpired(const payments::PaymentDetails& requestDetails);
+    // Verify the payment request amount is valid
+    static bool verifyAmount(const CAmount& requestAmount);
 
 signals:
     // Fired when a valid payment request is received