As you discovered,

int nines, predigit = 0;

is not the same as

int nines    = 0;
int predigit = 0;
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I'm surprised int nines, predigit = 0; is even valid C syntax.¹

However, nines, predigit = 0; is valid C syntax, but treats nines and predigit = 0 as 2 separate expressions. nines is evaluated in void context (so, the warning you got), while 0 is evaluated in an integer context and then assigned to predigit (effectively, predigit is in lvalue context).

An alternate syntax that would have zero'd both is nines = predigit = 0; which may be what the author intended to write (but he should have gotten the warning).

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¹ Note that in Perl, my ($nines, $predigit) = 0; will init $nines to 0 and leave $predigit undefined. Also, it doesn't give a warning.