Fix URL assertion failure found by fuzzing

This aligns canonicalizeOpaquePathname() with the other canonicalize
functions.
Also elides a superfluous memory allocation.

src/workerd/jsg/url.c++

@@ -614,8 +614,10 @@ kj::Maybe<kj::String> canonicalizeOpaquePathname(
  // @see https://wicg.github.io/urlpattern/#canonicalize-an-opaque-pathname
  if (pathname.size() == 0) return kj::str();
  auto str = kj::str("fake:", pathname);
- auto url = KJ_ASSERT_NONNULL(Url::tryParse(str.asPtr()));
- return kj::str(url.getPathname());
+ KJ_IF_SOME(url, Url::tryParse(str.asPtr())) {
+ return kj::str(url.getPathname());
+ }
+ return kj::none;
 }
 
 kj::Maybe<kj::String> canonicalizeSearch(
@@ -2151,8 +2153,7 @@ UrlPattern::Result<UrlPattern::Init> UrlPattern::processInit(
  auto str = kj::str(protocol, "://fake-url");
  return KJ_ASSERT_NONNULL(Url::tryParse(str.asPtr()));
  } else {
- auto str = kj::str("fake://fake-url");
- return KJ_ASSERT_NONNULL(Url::tryParse(str.asPtr()));
+ return KJ_ASSERT_NONNULL(Url::tryParse("fake://fake-url"_kj));
  }
  }
  })();