style: Avoid crashing when calling insertRule("@import ...") on a detached sheet.

This should unblock the fuzzers for now, though it's not the ideal solution.

It's the only reasonably easy solution to unblock them though, I think.

We should probably always keep track of the document a stylesheet was associated
with. We'll need that for constructible stylesheets anyway.

That requires some though on how to get the cycle-collection and such right,
though, and I wouldn't be able to write or land that ASAP.

Differential Revision: https://phabricator.services.mozilla.com/D23584

components/style/stylesheets/rule_parser.rs

@@ -189,6 +189,13 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                     return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedImportRule))
                 }
 
+                // FIXME(emilio): We should always be able to have a loader
+                // around! See bug 1533783.
+                if self.loader.is_none() {
+                    error!("Saw @import rule, but no way to trigger the load");
+                    return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedImportRule))
+                }
+
                 let url_string = input.expect_url_or_string()?.as_ref().to_owned();
                 let url = CssUrl::parse_from_string(url_string, &self.context);