Fix potential issues when determining application working directory on startup

[seer-by-sentry]

Fixes CLIENT-1HJ. The issue was that: Unvalidated strrchr result during working directory setup causes NULL pointer dereference, corrupting stack before GetFullPathNameW call.

• Improved robustness when using GetModuleFileName to determine the application's directory path in various WinMain implementations.
• The path buffer is now explicitly initialized to null.
• The return value of GetModuleFileName is checked to ensure the call succeeded and the path was not truncated.
• SetCurrentDirectory is only called if the module file name retrieval and path parsing were successful.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 8131099

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