Errors that occur when deleting release

• Gitea version (or commit ref): f54626d (After Fix some webhooks bugs #3981)
• Git version: 2.16.3
• Operating system: macOS
• Database:
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (I think yes, because the page is not refreshed)
  • No
  • Not relevant
• Log gist:

[Macaron] PANIC: runtime error: invalid memory address or nil pointer dereference
/usr/local/Cellar/go/1.10.2/libexec/src/runtime/panic.go:502 (0x402d248)
	gopanic: reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
/usr/local/Cellar/go/1.10.2/libexec/src/runtime/panic.go:63 (0x402c2bd)
	panicmem: panic(memoryError)
/usr/local/Cellar/go/1.10.2/libexec/src/runtime/signal_unix.go:388 (0x4043319)
	sigpanic: panicmem()
/Users/yasuokav/go/src/code.gitea.io/gitea/models/access.go:64 (0x4913ed6)
	accessLevel: if !repo.IsPrivate {
/Users/yasuokav/go/src/code.gitea.io/gitea/models/release.go:451 (0x4981de3)
	DeleteReleaseByID: mode, _ := accessLevel(x, u.ID, rel.Repo)
/Users/yasuokav/go/src/code.gitea.io/gitea/routers/repo/release.go:288 (0x4ad52fd)
	DeleteRelease: if err := models.DeleteReleaseByID(ctx.QueryInt64("id"), ctx.User, true); err != nil {

Description

As title. The release will be deleted, but the attachments did not delete(this is another issue #2618).

Screenshots

N/A

[techknowlogick]

Closed with #4207