nmea: Fix conversion of leap second.

When a leap second is inserted, the RMC message reports time of
23:59:60, which overflows in mktime() to 0:00:00 after the leap second
with an incremented TAI-UTC offset. This causes a one-second error in
the offset meaured by ts2phc.

Check the seconds field of the RMC message and convert 60 to 59 to make
the timestamp ambiguous per is_utc_ambiguous() and ignored by ts2phc to
avoid updating the clock with the one-second error.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>

nmea.c

@@ -155,6 +155,9 @@ static int nmea_scan_rmc(struct nmea_parser *np, struct nmea_rmc *result)
 	if (cnt != 3) {
 		return -1;
 	}
+	/* Convert an inserted leap second to ambiguous 23:59:59 */
+	if (tm.tm_sec == 60)
+		tm.tm_sec = 59;
 	tm.tm_year += 100;
 	tm.tm_mon--;
 	tm.tm_isdst = 0;