Missile guidance scripts do not remove the other bonus

[oskar94]

After doing some fitting with MGC's I noticed that even when I scriped for precision the range changed. By looking at the 'Affected by' tab on the missiles i could see that the stats of the MGC increased when the script was loaded but the other bonus was still the same as with no scrip in.

[blitzmann]

This is a duplicate of #406, in which 0-valued modifiers don't work correctly. It's been fixed in the repo. =)