Handle time out of range in libx52_set_clock

Prior to this change, the assumption was that localtime/gmtime would
never fail, regardless of the time value provided to it. However,
testing on an Ubuntu 20.04 machine revealed that the representable
range of time_t was about 56 bits, values that exceed a 56 bit
representation would cause localtime/gmtime to return a NULL pointer.

This change replaces the use of localtime/gmtime with their
corresponding thread-safe variants, and checks the return value against
NULL. If it matches a NULL value, then it will return an error and not
update the clocks.

ChangeLog.md

@@ -21,6 +21,7 @@ The format is based upon [Keep a Changelog].
 
 ### Fixed
 - Error reporting in x52cli and x52test commands.
+- Handling of very large time_t values in `libx52_set_clock`
 
 ## [0.2.0] - 2020-04-14
 ### Changed

lib/libx52/libx52.h

@@ -178,7 +178,7 @@ typedef enum {
     /** No change necessary, try again */
     LIBX52_ERROR_TRY_AGAIN,
 
-    /** Clock timezone out of range */
+    /** Clock time value or timezone out of range */
     LIBX52_ERROR_OUT_OF_RANGE,
 
     /** Error encountered during USB interaction */

lib/libx52/x52_date_time.c

@@ -19,6 +19,7 @@
 int libx52_set_clock(libx52_device *x52, time_t time, int local)
 {
     struct tm timeval;
+    struct tm *ptr;
     int local_tz;
     int local_date_day;
     int local_date_month;
@@ -32,17 +33,24 @@ int libx52_set_clock(libx52_device *x52, time_t time, int local)
     }
 
     if (local) {
-        timeval = *localtime(&time);
+        ptr = localtime_r(&time, &timeval);
         /* timezone from time.h presents the offset in seconds west of GMT.
          * Negate and divide by 60 to get the offset in minutes east of GMT.
          */
         local_tz = (int)(-timezone / 60);
     } else {
-        timeval = *gmtime(&time);
+        ptr = gmtime_r(&time, &timeval);
         /* No offset from GMT */
         local_tz = 0;
     }
 
+    if (ptr == NULL) {
+        /* Time conversion failed. This happens if the time value exceeds the
+         * range which can be represented.
+         */
+        return LIBX52_ERROR_OUT_OF_RANGE;
+    }
+
     local_date_day = timeval.tm_mday;
     local_date_month = timeval.tm_mon + 1;
     local_date_year = timeval.tm_year % 100;