Fix hostname validation bug in relative URL parser

The relative URL parser allowed certain hosts which the absolute parser correctly weeded out. This change copies two lines from the absolute parser over to the relative parser.

c++/src/kj/compat/url-test.c++

@@ -371,5 +371,10 @@ KJ_TEST("parse relative URL") {
                         "https://capnproto.org/http%3A/grault");
 }
 
+KJ_TEST("parse relative URL failure") {
+  auto base = Url::parse("https://example.com/");
+  KJ_EXPECT(base.tryParseRelative("https://[not a host]") == nullptr);
+}
+
 }  // namespace
 }  // namespace kj

c++/src/kj/compat/url.c++

@@ -279,6 +279,8 @@ Maybe<Url> Url::tryParseRelative(StringPtr text) const {
     }
 
     result.host = percentDecode(authority, err);
+    if (!HOST_CHARS.containsAll(result.host)) return nullptr;
+    toLower(result.host);
   } else {
     // copy authority
     result.host = kj::str(this->host);