The Debian packaging of smex is maintained in git, using the merging
workflow described in dgit-maint-merge(7).  There isn't a patch queue
that can be represented as a quilt series.

A detailed breakdown of the changes is available from their canonical
representation - git commits in the packaging repository.  For
example, to see the changes made by the Debian maintainer in the first
upload of upstream version 1.2.3, you could use:

    % git clone https://git.dgit.debian.org/smex
    % cd smex
    % git log --oneline 1.2.3..debian/1.2.3-1 -- . ':!debian'

(If you have dgit, use `dgit clone smex`, rather than plain `git
clone`.)

A single combined diff, containing all the changes, follows.
--- smex-3.0.orig/smex.el
+++ smex-3.0/smex.el
@@ -240,8 +240,8 @@ Set this to nil to disable fuzzy matchin
             (error (if (smex-save-file-not-empty-p)
                        (error "Invalid data in smex-save-file (%s). Can't restore history."
                               smex-save-file)
-                     (if (not (boundp 'smex-history)) (setq smex-history))
-                     (if (not (boundp 'smex-data))    (setq smex-data))))))
+                     (unless (boundp 'smex-history) (setq smex-history nil))
+                     (unless (boundp 'smex-data)    (setq smex-data nil))))))
       (setq smex-history nil smex-data nil))))
 
 (defsubst smex-save-file-not-empty-p ()
