Fix AI targeting itself during EMP

[JohnAFernandez]

To keep the AI code from going crazy, we should keep the AI from targeting itself during/after EMP. The AI will attempt to do this if they are set to traitor.

Closes #3366

[Goober5000]

This is good, but as a matter of code style I think it should be the second condition, after (ship_lookup >= 0). Intuitively, checking that the ship isn't itself would be the second thing to look for, right after checking for a valid index.

[JohnAFernandez]

This is good, but as a matter of code style I think it should be the second condition, after (ship_lookup >= 0). Intuitively, checking that the ship isn't itself would be the second thing to look for, right after checking for a valid index.

I disagree. Because the program will follow the same order of checks as it does in the code, and this is the rarest case of it being rejected, this is the most efficient way to add the check.