Fix Null Pointer dereference with proper return.

In reference to https://github.com/libwbxml/libwbxml/commit/9f7fb3008fc0156e8bfd12c8e4cb41c70e315c68
wbxml_list_get can return NULL.
In that case parse_attribute will crash as it dereference result of wbxml_list_get which is encode bug and hence returns WBXML_ERROR_INTERNAL.
So this fix is applied.