Typo bug in concatenate-source

In code intended to populate an a-list nil is accidentally used as the key instead of the actual value. This looks like a typo which doesn't trigger serious functional problems (just a less meaningful warning).

modified   concatenate-source.lisp
@@ -72,7 +72,7 @@ into a single file"))
               (unless (equal e encoding)
                 (let ((a (assoc e other-encodings)))
                   (if a (push (component-find-path c) (cdr a))
-                      (push (list a (component-find-path c)) other-encodings)))))
+                      (push (list e (component-find-path c)) other-encodings)))))
             (unless (equal around-compile (around-compile-hook c))
               (push (component-find-path c) other-around-compile))
             (input-files (make-operation 'compile-op) c)) :into inputs

Thanks, Eric