MM-36893 : Specially crafted message request crashes the webapp for users who view the message (#18835)

* strip away delete

* added tests

api4/post.go

@@ -43,6 +43,9 @@ func createPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Strip away delete_at if passed
+	post.DeleteAt = 0
+
 	post.UserId = c.AppContext.Session().UserId
 
 	auditRec := c.MakeAuditRecord("createPost", audit.Fail)