Handle deleted base branch in PR (#10618)

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

routers/repo/pull.go

@@ -356,6 +356,16 @@ func PrepareViewPullInfo(ctx *context.Context, issue *models.Issue) *git.Compare
 		return nil
 	}
 	defer baseGitRepo.Close()
+
+	if !baseGitRepo.IsBranchExist(pull.BaseBranch) {
+		ctx.Data["IsPullRequestBroken"] = true
+		ctx.Data["BaseTarget"] = pull.BaseBranch
+		ctx.Data["HeadTarget"] = pull.HeadBranch
+		ctx.Data["NumCommits"] = 0
+		ctx.Data["NumFiles"] = 0
+		return nil
+	}
+
 	var headBranchExist bool
 	var headBranchSha string
 	// HeadRepo may be missing