Gogs has DoS in rendering issue index pattern

Description

### Summary Special template of issue index pattern may cause panic. ### Details in internal/markup/markup.go ```go link = fmt.Sprintf(`<a href="%s">%s</a>`, com.Expand(metas["format"], metas), m) ``` Issue index pattern is rendered to link with `com.Expand`. However, `com.Expand` is not safe. ```go i = strings.Index(template, "}") if s, ok := match[template[:i]]; ok { ``` when `{` is found but `}` not found, i comes to 1, template[:-1] will be called, and then panicked  finally, all pages than contains issue index are unavailable. ### PoC 1. set issue index pattern as follow  2. add a commit which point to an issue in its msg  using `#1` above ### Impact DoS that cause part of pages of the specify repo unavailable.

Affected Packages (1)