Fix URLs not getting substituted after download

That is, on the first run of MIRROR-IMG, no
mirrored URLs got substituted — and on subsequent
runs, only previously-downloaded URLS did.

src/mirror-img.lisp

@@ -139,7 +139,8 @@ already set)."
 (defun http-fetch (url path)
   "Download a URL to a path; if successful, returns the pathname. Otherwise, NIL."
   (handler-case
-      (and (dexador:fetch (url-encode-uri url) path)
+      ; DEXADOR:FETCH returns nil on success, errors on fail. Hence the OR.
+      (or (dexador:fetch (url-encode-uri url) path)
           (pathname path))
     (cl-user::file-exists ()
       path)