- fixed_ P_ExplodeMissile skipped most of its logic when the missile got destroyed by having no death state.

Apparently a band-aid fix was applied to this function because AActor::Destroy could not be called on an already destroyed actor again which could happen here due to incorrect ordering of actions.
Moving the state change to a later point in this function - after hitting a sky has been checked and decals have been spawned - returns everything to a safe state and ensures that nothing gets skipped.
- some parentheses to silence GCC.