dired-rsync: don't trigger errors in filter function

`search-backward' can fail if there is no \r to find which seems to
happen in some cases. Handle the failure more gracefully.

Fixes: 49

dired-rsync.el

@@ -270,7 +270,7 @@ information and update the dired-rsync-modeline-status."
             (set-marker (process-mark proc) (point))
             ;; Delete old text upto the newline
             (goto-char (point-max))
-            (when (search-backward "\r")
+            (when (search-backward "\r" nil t)
               (delete-region (point-min) (+ 1 (match-beginning 0)))))
           (if moving
               (goto-char (process-mark proc))))))))