Missing error check in base64 encoder

[mtausig]

The function b64encode of the source file utils_win.cpp [does not check the return value] (https://github.com/haiwen/seadrive-gui/blob/8a1e459f1c42a24eec6fcce482398bebf6459e1c/src/utils/utils-win.cpp#L349) of CryptBinaryToString. A possible reason for an error returned would be, if the length of input is longer than 32 KiB.

Thus an error goes unnoticed in every function calling b64encode.

The return value of CryptBinaryToString should be checked and an error propagated to every calling function, which in turn needs to add a check.