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>