ensure proper range for argument to ctype functions

lib/dns/ttl.c

@@ -109,6 +109,9 @@ dns_ttl_totext(isc_uint32_t src, isc_boolean_t verbose, isc_buffer_t *target) {
 		/*
 		 * The unit letter is the last character in the 
 		 * used region of the buffer.
+		 *
+		 * toupper() does not need its argument to be masked of cast
+		 * here because region.base is type unsigned char *.
 		 */
 		isc_buffer_usedregion(target, &region);
 		region.base[region.length - 1] =