Update ImGuiFileDialog.cpp

[ddddhm1234]

On Windows systems, when using file_dialog to browse a directory without access permission, such as C:\Users<your user>\SendTo, dpg would hang or crash. This PR fixes this bug.

Operating system:
Windows

Reproduce:
Use file_dialog to browse any directory without access permission on Windows

The origin of this bug:
In ImGuiFileDialog.cpp, ScanDir function

//const auto wpath = IGFD::Utils::WGetString(path.c_str());
const std::filesystem::path fspath(path);
const auto dir_iter = std::filesystem::directory_iterator(fspath); //  <--------  If the current user doesn't have permission to access this directory, an exception will be thrown here without a try block to handle it.