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: NRichard Levitte <levitte@openssl.org>

crypto/stack/stack.c

@@ -299,7 +299,7 @@ void sk_zero(_STACK *st)
         return;
     if (st->num <= 0)
         return;
-    memset((char *)st->data, 0, sizeof(st->data) * st->num);
+    memset((char *)st->data, 0, sizeof(*st->data) * st->num);
     st->num = 0;
 }