bltins/enum.c: Fix integer truncation in `put_enum` (#241)

This bugfix comes from <https://github.com/att/ast/pull/711>.
Eric Scrivner provided the following explanation for the fix:

> Coverity identified an issue with integer truncation in
> `put_enum`. The function was truncating the return values of
> `strcasecmp` and `strcmp` from an `int` to an `unsigned short`
> when assigning them to the local variable `n`. Since either of
> these methods can return a value that is not in the set `{0, 1,
> -1}` the later check if `n == 0` could spuriously evaluate to
> true. For example, in the case where either function returned
> `-65536`.

> The fix is simply to change `n` from an `unsigned short` to an
> `int` to avoid the possibility of truncation. Since the only
> purpose of `n` is the store the return values of these checks,
> this does not have any side effects.

src/cmd/ksh93/bltins/enum.c

@@ -121,7 +121,8 @@ static void put_enum(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 {
 	struct Enum 		*ep = (struct Enum*)fp;
 	register const char	*v;
-	unsigned short		i=0, n;
+	unsigned short		i=0;
+	int			n;
 	if(!val)
 	{
 		nv_putv(np, val, flags,fp);