handle errors from localtime_r in ctime_r

POSIX requires ctime_r return a null pointer on failure, which can
occur if the input time_t value is not representable in broken down
form.

based on patch by Alexander Monakov.

src/time/ctime_r.c

@@ -2,7 +2,6 @@
 
 char *ctime_r(const time_t *t, char *buf)
 {
-	struct tm tm;
-	localtime_r(t, &tm);
-	return asctime_r(&tm, buf);
+	struct tm tm, *tm_p = localtime_r(t, &tm);
+	return tm_p ? asctime_r(tm_p, buf) : 0;
 }