in_tail: Add a missing nil check for rotated_tw in update_watcher

The tail watcher for the path might by already removed from @Tails by
refresh_watchers.

Fix #3752

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

lib/fluent/plugin/in_tail.rb

@@ -511,7 +511,7 @@ def update_watcher(target_info, pe)
         if new_position_entry.read_inode == 0
           # When follow_inodes is true, it's not cleaned up by refresh_watcher.
           # So it should be unwatched here explicitly.
-          rotated_tw.unwatched = true
+          rotated_tw.unwatched = true if rotated_tw
           @tails[path] = setup_watcher(target_info, new_position_entry)
           @tails[path].on_notify
         end