[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