Fix panic in api4.searchPosts (#15410)

api4/post.go

@@ -466,7 +466,11 @@ func searchPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	params := model.SearchParameterFromJson(r.Body)
+	params, jsonErr := model.SearchParameterFromJson(r.Body)
+	if jsonErr != nil {
+		c.Err = model.NewAppError("searchPosts", "api.post.search_posts.invalid_body.app_error", nil, jsonErr.Error(), http.StatusBadRequest)
+		return
+	}
 
 	if params.Terms == nil || len(*params.Terms) == 0 {
 		c.SetInvalidParam("terms")