[MM-28383] Fix nil pointer dereference in addTeamMember (#15422)

* Fix nil pointer dereference in addTeamMember

* Revert to simple nil check

* Add test case

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/team.go

@@ -569,6 +569,10 @@ func addTeamMember(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	var err *model.AppError
 	member := model.TeamMemberFromJson(r.Body)
+	if member == nil {
+		c.Err = model.NewAppError("addTeamMember", "api.team.add_team_member.invalid_body.app_error", nil, "Error in model.TeamMemberFromJson()", http.StatusBadRequest)
+		return
+	}
 	if member.TeamId != c.Params.TeamId {
 		c.SetInvalidParam("team_id")
 		return

api4/team_test.go

@@ -1847,6 +1847,11 @@ func TestAddTeamMember(t *testing.T) {
 	_, resp = Client.AddTeamMember(team.Id, otherUser.Id)
 	CheckNoError(t, resp)
 
+	// Should return error with invalid JSON in body.
+	_, err = Client.DoApiPost("/teams/"+team.Id+"/members", "invalid")
+	require.NotNil(t, err)
+	require.Equal(t, "api.team.add_team_member.invalid_body.app_error", err.Id)
+
 	// by token
 	Client.Login(otherUser.Email, otherUser.Password)
 

i18n/en.json

@@ -2182,6 +2182,10 @@
     "id": "api.team.add_members.user_denied",
     "translation": "This team is managed by groups.  This user is not part of a group that is synced to this team."
   },
+  {
+    "id": "api.team.add_team_member.invalid_body.app_error",
+    "translation": "Unable to parse the request body."
+  },
   {
     "id": "api.team.add_user_to_team.added",
     "translation": "%v added to the team by %v."