check that $function is callable

Closes #3432388

filefield_sources.module

@@ -643,7 +643,9 @@ function filefield_sources_element_validate($element, $file, FormStateInterface
       // reference (required for PHP 5.3 and higher).
       array_unshift($args, NULL);
       $args[0] = &$file;
-      $errors = array_merge($errors, call_user_func_array($function, $args));
+      if (is_callable($function)) {
+        $errors = array_merge($errors, call_user_func_array($function, $args));
+      }
     }
   }