getdate+nts+rtc: avoid some coverity false positives

Modify the code to avoid making the following calls incorrectly reported
as important findings by the coverity static analyzer:

- memset() of size 0 at the end of an array
- mktime() on a struct tm that has uninitialized tm_yday

rtc_linux.c

@@ -359,13 +359,13 @@ t_from_rtc(struct rtc_time *rtc_raw, int utc)
   time_t t1, t2;
 
   /* Convert to seconds since 1970 */
+  memset(&rtc_tm, 0, sizeof (rtc_tm));
   rtc_tm.tm_sec = rtc_raw->tm_sec;
   rtc_tm.tm_min = rtc_raw->tm_min;
   rtc_tm.tm_hour = rtc_raw->tm_hour;
   rtc_tm.tm_mday = rtc_raw->tm_mday;
   rtc_tm.tm_mon = rtc_raw->tm_mon;
   rtc_tm.tm_year = rtc_raw->tm_year;
-  rtc_tm.tm_wday = 0;
 
   temp1 = rtc_tm;
   temp1.tm_isdst = 0;