Fix the protected branch panic issue (#3567) (#3569)

Signed-off-by: Wendell Sun <iwendellsun@gmail.com>

models/branches.go

@@ -156,6 +156,10 @@ func (repo *Repository) GetProtectedBranches() ([]*ProtectedBranch, error) {
 
 // IsProtectedBranch checks if branch is protected
 func (repo *Repository) IsProtectedBranch(branchName string, doer *User) (bool, error) {
+	if doer == nil {
+		return true, nil
+	}
+
 	protectedBranch := &ProtectedBranch{
 		RepoID:     repo.ID,
 		BranchName: branchName,