Fix an off-by-one error in cfg_print_duration()

The cfg_print_duration() checks added previously in the 'duration_test'
unit test uncovered a bug in cfg_print_duration().

When calculating the current 'str' pointer of the generated text in the
buffer 'buf', it erroneously adds 1 byte to compensate for that part's
indicator character. For example, to add 12 minutes, it needs to add
2 + 1 = 3 characters, where 2 is the length of "12", and 1 is the length
of "M" (for minute). The mistake was that the length of the indicator
is already included in 'durationlen[i]', so there is no need to
calculate it again.

In the result of this mistake the current pointer can advance further
than needed and end up after the zero-byte instead of right on it, which
essentially cuts off any further generated text. For example, for a
5 minutes and 30 seconds duration, instead of having this:

    'P', 'T', '5', 'M', '3', '0', 'S', '\0'

The function generates this:

    'P', 'T', '5', 'M', '\0', '3', '0', 'S', '\0'

Fix the bug by adding to 'str' just 'durationlen[i]' instead of
'durationlen[i] + 1'.

lib/isccfg/parser.c

@@ -1096,7 +1096,7 @@ cfg_print_duration(cfg_printer_t *pctx, const cfg_obj_t *obj) {
 		if (duration.parts[i] > 0) {
 			snprintf(str, durationlen[i] + 2, "%u%c",
 				 (uint32_t)duration.parts[i], indicators[i]);
-			str += durationlen[i] + 1;
+			str += durationlen[i];
 		}
 		if (i == 3 && T) {
 			snprintf(str, 2, "T");