Panic when listing user repos using API and not authenticated

[cez81]

• Gitea version (or commit ref): 1.1.0+138-gc58708d3
• Git version: 2.7.4
• Operating system: Ubuntu 16.04
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL) https://try.gitea.io/api/v1/users/cez81/repos
  • No
  • Not relevant
• Log gist:

/home/jonas/go/src/code.gitea.io/gitea/routers/api/v1/user/repo.go:12 (0x101b389)
	listUserRepos: showPrivateRepos := (ctx.User.ID == userID || ctx.User.IsAdmin) && ctx.IsSigned
/home/jonas/go/src/code.gitea.io/gitea/routers/api/v1/user/repo.go:40 (0x101b764)
	ListUserRepos: listUserRepos(ctx, user)

Description

Trying to get users repositories using the API when not signed in will cause panic on line https://github.com/go-gitea/gitea/blob/master/routers/api/v1/user/repo.go#L12.
ctx.User is not populated unless signed in.

Github seems to allow this operation even when nog logged in so I guess that should be correct behaviour?

[bkcsoft]

Related to #1539 which has the same issue (but for the web-view controller)