Check if users exists before adding them to a conversation

src/controllers/conversations_controller.rs

@@ -84,6 +84,13 @@ pub fn update_settings(r: &mut HttpRequestHandler) -> RequestResult {
             r.forbidden("You can not update the list of members of this conversation!".to_string())?;
         }
 
+        // Check if the members of the conversation really exists
+        for member in &members {
+            if !user_helper::exists(member.clone())? {
+                r.not_found(format!("User {} not found!", member))?;
+            }
+        }
+
         if !members.contains(&r.user_id()?) {
             members.push(r.user_id()?);
         }