doc-view: Fix error during revert in editing mode

doc-view: Fix error during revert in editing mode

When in editing mode in a doc-view buffer and then reverting (which can
happen automatically when editing OpenDocument contents), we errored in
the advide function doc-view--revert-buffer because the local
doc-view--buffer-file-name has been killed when switching to the editing
mode.

Also restore doc-view-minor-mode after reverting during being in the
editing mode.

* lisp/doc-view.el (doc-view--revert-buffer): Check that
'doc-view--buffer-file-name' is non-nil.
(doc-view-minor-mode): Add re-enabling function to
'revert-buffer-restore-functions'.

(cherry picked from commit 34362a2a1ccd229b486d87cbf4de2eed66e0010d)