src/svgtiny_css.c: fix uninitalized use in node_is_lang()

src/svgtiny_css.c: fix uninitalized use in node_is_lang()

If _node_has_attribute_substring() returns an error before it has a
chance to overwrite the "match" variable with true/false, then we may
try to access "match" while it is uninitialized. We may as well just
initialize it to false (no match) to avoid this corner case.