Fix Possible Null Pointer De-reference.

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.
So this fix is applied.