avoid repeated tzset() calls

The  tzset()  function  initializes  the tzname variable from the
TZ environment variable.

There doesn't seem to be any valid usecase of calling this repeatedly.
It is an overhead that can be avoided.

Author: irfanHaslanded

src/tree_data_common.c

@@ -1620,14 +1620,26 @@ ly_time_tz_offset(void)
     return ly_time_tz_offset_at(time(NULL));
 }
 
+static void
+ly_tzset(void)
+{
+    static int ly_tzset_called = 0;
+
+    if (ly_tzset_called) {
+        return;
+    }
+    tzset();
+    ly_tz_set_called = 1;
+}
+
 LIBYANG_API_DEF int
 ly_time_tz_offset_at(time_t time)
 {
     struct tm tm_local, tm_utc;
     int result = 0;
 
     /* init timezone */
-    tzset();
+    ly_tzset();
 
     /* get local and UTC time */
     localtime_r(&time, &tm_local);
@@ -1777,7 +1789,7 @@ ly_time_time2str(time_t time, const char *fractions_s, char **str)
     LY_CHECK_ARG_RET(NULL, str, LY_EINVAL);
 
     /* init timezone */
-    tzset();
+    ly_tzset();
 
     /* convert */
     if (!localtime_r(&time, &tm)) {