Fix memset call in stack.c

The function sk_zero is supposed to zero the elements held within a stack.
It uses memset to do this. However it calculates the size of each element
as being sizeof(char **) instead of sizeof(char *). This probably doesn't
make much practical difference in most cases, but isn't a portable
assumption.

Reviewed-by: Richard Levitte <levitte@openssl.org>
