`obj_get_signature()` examines all objects with a type other than
`OBJ_NONE` to find an unused signature.
`load_game_data()`->`read_object()` set an object's type before calling
`obj_get_signature()`, so `obj_get_signature()` would consider the
uninitialized signature of the newly loaded object for exclusion.
Reorder the initialization to compute the signature before the object is
given a type, then store the signature on the object after the poison
bytes (if any) are written.
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