Avoid warnings about undo list while decoding

Avoid warnings about undo list while decoding

* src/coding.c (coding_restore_undo_list): New function.
(decode_coding_gap, decode_coding_object): Temporarily set the
buffer's undo list to t, to avoid warnings when decoding changes
the buffer many times, e.g. due to post-read-conversion.  Use
coding_restore_undo_list to make sure the undo list is restored.
(Bug#23813)