If an integer overflow is about to be reached, call xalloc_err_handler()
(since it would have been caught by calloc() or reallocarray()) and then
exit(): these integer overflows are far from reachable, with the current
PID_MAX_LIMIT (2^22), so if they are there is something very wrong going
on. Note: we check the n_*alloc variables against INT_MAX even when they
are size_t because they are later stored as int in a struct proc_data_t.
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