Improve oslLoadImageFile function for better safety and extensibility

[dogo]

• Adjusted the loop condition to use sizeof(extension) - 1 instead of -2, allowing the full extension to be copied while still ensuring space for a null terminator.
• Initialized the extension buffer with zeros, removing the need for manual null termination after the loop.
• Ensured the code handles edge cases such as the extension being at the start of the filename.