Limit url to local path

[stonezdj]

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[wy65701436]

lgtm

[pborn-ionos]

This seems to break Harbor for us. When you're on /account/sign-in with OIDC, the login button for OIDC points to /c/oidc/login?redirect_url= and an empty redirect_url errors out with

{
  "errors": [
    {
      "code": "BAD_REQUEST",
      "message": "cannot redirect to other site"
    }
  ]
}