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As long as snprintf is used with a fixed size buffer using an idiom
 snprintf(buffer, sizeof(buffer), ..)
there is no issue because sizeof(buffer) cannot be zero. AFAICT, this 
is how python uses PyOS_vsnprintf wrapper.
On the other hand, may this is a good opportunity to revisit the 
decision to make PyOS_vsnprintf semantics different from C99 vsnprintf.
C99 defines snprintf semantics as follows:
int snprintf(char *restrict s, size_t n,
 const char *restrict format, ...);
The snprintf() function shall be equivalent to sprintf(), with the 
addition of the n argument which states the size of the buffer referred 
to by s. If n is zero, nothing shall be written and s may be a null 
pointer. Otherwise, output bytes beyond the n-1st shall be discarded 
instead of being written to the array, and a null byte is written at the 
end of the bytes actually written into the array.
<http://www.opengroup.org/onlinepubs/000095399/functions/printf.html>