Fix addon MD5 buffer overflow

[tulpenkiste]

The original code was using sizeof(so) which, on my machine, is returning 8. This causes the game to try to write to a point past so[32], causing a buffer overflow.
This PR simply hardcodes the size to 3 (as each char is always 1 byte large, theres 2 chars inserted via the format + 1 NULL char). This makes it not buffer overflow without breaking existing addon functionality.

[MatusGuy]

Is this a duplicate of #3136?

[tulpenkiste]

Ah, looks like it is, although Semphriss's explanation is wrong

[swagtoy]

This is indeed a duplicate, and I complained about our MD5 code before. But for now, if we want to fix this issue, we can just merge this. Semphris' code has an issue with the comments and he hasn't responded. I don't personally care about refactoring right now if this is an issue.