gadget.lua: not crash when Explosion_GetWantedWeaponDef() not defined

same as https://github.com/ZeroK-RTS/Zero-K/commit/da8056b0ed5874dc8778c1d909b74247f8edb3aa commit. Will happen when map maker try to use Explosion() event.
Merge pull request #48 from xponen/patch-1

gadget.lua: not crash when Explosion_GetWantedWeaponDef() not defined