841 WeaponDefs is zero indexed

We only discovered this in ZK by cleaning out unused weapons. Now a weapon that appears in the game has weaponDefID 0 and it is causing issues. UnitDefs is one indexed.

```
for i, data in pairs(WeaponDefs) do
Spring.Echo("index", i, "weapon", data.name)
end

for i, data in pairs(UnitDefs) do
Spring.Echo("index", i, "unit", data.name)
end
...
[t=00:02:04.208972][f=0001965] index, 703, weapon, zenith_meteor_aim
[t=00:02:04.208998][f=0001965] index, 704, weapon, zenith_meteor_float
[t=00:02:04.209024][f=0001965] index, 705, weapon, zenith_meteor_uncontrolled
[t=00:02:04.209068][f=0001965] index, 0, weapon, amphaa_missile
```