Prevent NPE on update mirror settings (#19864)

A `repo_model.Mirror` repository field (`.Repo`) will not automatically
be set, but is used without checking in mirror_pull.go:UpdateAddress.
This will cause an NPE.

This PR changes UpdateAddress to use the helper function GetRepository()
helping prevent future NPEs but also changes modules/context/repo.go to
ensure that the Mirror.Repo is set.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

modules/context/repo.go

@@ -391,7 +391,7 @@ func repoAssignment(ctx *Context, repo *repo_model.Repository) {
 				ctx.ServerError("GetMirrorByRepoID", err)
 				return
 			}
-			ctx.Repo.Mirror.Repo = ctx.Repo.Repository
+			ctx.Repo.Mirror.Repo = repo
 			ctx.Data["MirrorEnablePrune"] = ctx.Repo.Mirror.EnablePrune
 			ctx.Data["MirrorInterval"] = ctx.Repo.Mirror.Interval
 			ctx.Data["Mirror"] = ctx.Repo.Mirror