Fix NetBSD ctype warnings: "array subscript has type 'char'"

As noted previously in commit e8f6d53, we have to cast to int via unsigned
char on NetBSD to ensure that the values fall into the correct range.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

digest.c

@@ -162,14 +162,14 @@ int digest_authorization(struct openconnect_info *vpninfo, struct oc_text_buf *h
 				chall = p;
 			}
 		}
-		while (isspace(*chall))
+		while (isspace((int)(unsigned char)*chall))
 			chall++;
 		if (!*chall)
 			break;
 		if (*chall != ',')
 			goto err;
 		chall++;
-		while (isspace(*chall))
+		while (isspace((int)(unsigned char)*chall))
 			chall++;
 		if (!*chall)
 			break;