src/svgtiny_css.c free dom_string in svgtiny_dom_user_data_handler()

src/svgtiny_css.c free dom_string in svgtiny_dom_user_data_handler()

This callback uses dom_string_create() for a comparison, but forgets
to call either dom_string_destroy() or dom_string_unref() after the
string has outlived its usefulness. The string is used one line after
it is created, so we find dom_string_destroy() to be more clear in
this case.