libc: Fix mktime returns an uncorrect time in empty TZ case

The mktime API returned an uncorrect time when TZ is set as empty.
A timezone UTC/GMT+0 should be implied in the empty case. However
mktime keeps previous information about timezone. If mktime was called
with a timezone which has DST before, the "defaulttype" member of
"state" structure wouldn't be 0. Then it would be used next time,
even though UTC/GMT+0 doesn't have DST.

Added initialization of the "defaulttype" in the empty TZ case.

Change-Id: Ic480c63c548c05444134e0aefb30a7b380e3f40b

libc/tzcode/localtime.c

@@ -1257,6 +1257,7 @@ tzset_locked(void)
         lclptr->ttis[0].tt_gmtoff = 0;
         lclptr->ttis[0].tt_abbrind = 0;
         (void) strcpy(lclptr->chars, gmt);
+        lclptr->defaulttype = 0;
     } else if (tzload(name, lclptr, TRUE) != 0)
         if (name[0] == ':' || tzparse(name, lclptr, FALSE) != 0)
             (void) gmtload(lclptr);