Fix checking for alphanumeric characters using proper conversion to a void undefined behavior (and assert message with MSBuild)

[Robxley]

Hi,

When I use font icons in the name of a dockable window, such as:

HelloImGui::DockableWindow windows;
windows.label = U8_TO_CHAR(ICON_FA_ROCKET " Unit tests");
windows.dockSpaceName = "MAIN_SPACE";
windows.GuiFunction = [&] { GuiFunction(); };

This allows me to display a window tab with an icon for better readability:

However, closing the application triggers an assertion error (with MSBuild) related to the isalnum function (observed in debug mode). The issue seems linked to undefined behavior caused by passing a char directly to isalnum, which expects an unsigned char (or EOF) as int.
Reference: cppreference.com on isalnum.
An example from cppreference.com suggests using static_cast<unsigned char> to pass a char from std::string to isalnum.

Stack Trace :

Steps to Reproduce (at least with MSBuild)

1. Create a dockable window using a label containing an icon (e.g., ICON_FA_ROCKET).
2. Close the application while in debug mode.
3. Observe the assertion error from isalnum.

[pthom]

Hi,

Thanks a lot for your PR and the thorough analysis !
I will merge your PR.

The CI errors are unrelated (they are due to vcpkg, which recently broke a loosely related package, see microsoft/vcpkg#43099)