Function `object_create_explosion_sub` is supposed to be given a weapon
and an object on which to explode it. However, the original game abused
`object_create_explosion_sub` by also calling it from
`object_create_badass_explosion`, which can be called from
`explode_badass_object`, which can be called from
`explode_badass_player`. Chained together, this lets
`object_create_explosion_sub` check the weapon ID of a dying player,
which is not a valid operation. This causes a diagnostic from
`get_weapon_id`. Add an explicit test that the object is a weapon so
that non-weapons do not cause a diagnostic.
Reported-by: ryusei117 <https://github.com/dxx-rebirth/dxx-rebirth/issues/302#issuecomment-272048330>
Analyzed-by: zicodxx <https://github.com/dxx-rebirth/dxx-rebirth/issues/302#issuecomment-272710279>
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