style: Check iterator length in SelectorIter::is_featureless_host_selector.

I'm going to unconditionally generate the PseudoElement combinator, and this
causes issues since we'll put the raw `::pseudo` selectors in the host bucket,
which is obviously wrong.

Differential Revision: https://phabricator.services.mozilla.com/D27528

components/selectors/parser.rs

@@ -736,6 +736,7 @@ impl<'a, Impl: 'a + SelectorImpl> SelectorIter<'a, Impl> {
     /// combinators to the left.
     #[inline]
     pub(crate) fn is_featureless_host_selector(&mut self) -> bool {
+        self.selector_length() > 0 &&
             self.all(|component| matches!(*component, Component::Host(..))) &&
             self.next_sequence().is_none()
     }