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calloc() allows 'nmemb' or 'size' to be zero, in which case NULL may be
returned. Letting htop die because of either argument being zero doesn't
make sense.

As a side note: As size_t is unsigned, compiler should be able to optimize
conditional (nmemb > 0 && size > 0) to (nmemb && size). This theorically
shouldn't increase code size too much.