[mod_webdav] fix startup crash w/ multiple conds (fixes #2958)

(thx flynn)

x-ref:
  "lighttpd 1.4.54 segfaults on start in mod_webdav"
  https://redmine.lighttpd.net/issues/2958

src/mod_webdav.c

@@ -1275,8 +1275,8 @@ SERVER_FUNC(mod_webdav_worker_init)
      *   across a fork() system call into the child process.
      */
     plugin_data * const p = (plugin_data *)p_d;
-    plugin_config *s = p->config_storage[0];
-    for (int n_context = p->nconfig+1; --n_context; ++s) {
+    for (int i = 0; i < p->nconfig; ++i) {
+        plugin_config *s = p->config_storage[i];
         if (!buffer_is_empty(s->sqlite_db_name)
             && mod_webdav_sqlite3_prep(s->sql, s->sqlite_db_name, srv->errh)
                == HANDLER_ERROR)