Don't crash if /var/tmp doesn't exist

`xstat` is checked `stat64` crashing the program if the latter returns
failure. In this loop, we are trying to find one folder that satisfies
the condition, no reason to crash the program if one folder doesn't.

io/tst-copy_file_range.c

@@ -759,8 +759,7 @@ do_test (void)
           }
 
         struct stat64 cstat;
-        xstat (path, &cstat);
-        if (cstat.st_dev == instat.st_dev)
+        if (stat (path, &cstat) != 0 || cstat.st_dev == instat.st_dev)
           {
             free (to_free);
             continue;