--atime-preserve=replace: fix correctness and performance bugs

reported by Eric Blake in
<http://lists.gnu.org/archive/html/bug-tar/2011-03/msg00000.html>.
* src/compare.c (diff_file): Do not restore atime of size-zero files.
* src/create.c (dump_file0): Likewise.  Also, do not restore atime
when fd is zero, because that indicates a file we haven't opened.

src/compare.c

@@ -234,7 +234,8 @@ diff_file (void)
 	      else
 		read_and_process (&current_stat_info, process_rawdata);
 
-	      if (atime_preserve_option == replace_atime_preserve)
+	      if (atime_preserve_option == replace_atime_preserve
+		  && stat_data.st_size != 0)
 		{
 		  struct timespec atime = get_stat_atime (&stat_data);
 		  if (set_file_atime (diff_handle, chdir_fd, file_name, atime)
@@ -528,7 +529,7 @@ verify_volume (void)
   if (may_fail)
     WARN((0, 0,
 	  _("Verification may fail to locate original files.")));
-  
+
   if (!diff_buffer)
     diff_init ();
 

src/create.c

@@ -1797,6 +1797,7 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
 	      set_exit_status (TAREXIT_DIFFERS);
 	    }
 	  else if (atime_preserve_option == replace_atime_preserve
+		   && fd && (is_dir || original_size != 0)
 		   && set_file_atime (fd, parentfd, name, st->atime) != 0)
 	    utime_error (p);
 	}