Add missing return (#24288)

```release-note
NONE
```

server/channels/api4/post.go

@@ -652,6 +652,7 @@ func getPostThread(c *Context, w http.ResponseWriter, r *http.Request) {
 	// Either only fromCreateAt must be set, or both fromPost and fromCreateAt must be set
 	if fromPost != "" && fromCreateAt == 0 {
 		c.SetInvalidParam("if fromPost is set, then fromCreatAt must also be set")
+		return
 	}
 
 	direction := ""