Fatal error when taxonomy_options_list() tries to call an undefined callback function

Reading this again after a year, I prefer to close this as Won't fix, because of this reason (see #20):

A fatal error is the proper response here. There is no guarantee that the result of this function is going to be correct if the callback function does not exist. As a consequence, the consumers of this function might proceed and you will end up with inconsistent data.

If we commit this patch, we will change the expected output, because instead of custom callback, the default taxonomy_allowed_values function will be used even if the custom callback function is set:

-  $function = !empty($field['settings']['options_list_callback']) ? $field['settings']['options_list_callback'] : 'taxonomy_allowed_values';
+  if (!empty($field['settings']['options_list_callback']) && function_exists($field['settings']['options_list_callback'])) {
+    $function = $field['settings']['options_list_callback'];
+  }
+  else {
+    $function = 'taxonomy_allowed_values';
+  }

This is not ideal and it could lead to unexpected behavior. If anybody needs the patch, feel free to use it to prevent the error.

Thanks all for your effort here.