Fix a double-free in Windows Pageant.

Reported by Colin Harrison; occurred on the error path in which the
user clicks 'cancel' in the passphrase box.

windows/winpgnt.c

@@ -377,11 +377,11 @@ static void win_add_keyfile(Filename *filename)
                                 NULL, PassphraseProc, (LPARAM) &pps);
         passphrase_box = NULL;
 
-        sfree(err);
-
         if (!dlgret)
             goto done;		       /* operation cancelled */
 
+        sfree(err);
+
         assert(passphrase != NULL);
 
         ret = pageant_add_keyfile(filename, passphrase, &err);