libflake: fix assertion crash when malformed URL falls through to path scheme

When a URL like `github:nixos/nixpkgs/nixpkgs.git?ref=<hash>` (using
`ref` instead of `rev`) failed the github input scheme, it fell
through to `parsePathFlakeRefWithFragment` which constructed a `path:`
`ParsedURL` with an empty authority but a relative path. This violated
RFC 3986 section 3.3 (authority present requires path starting with
`/`), causing an assertion failure in `renderAuthorityAndPath` when
`PathInputScheme` tried to format the URL for an error message.

This commit only sets the authority on absolute paths. Relative paths
get `std::nullopt` for authority, which is the correct representation
per the URL spec.

Fixes #15196. Fixes #14830.

src/libflake-tests/flakeref.cc

@@ -283,4 +283,18 @@ TEST(to_string, doesntReencodeUrl)
     ASSERT_EQ(unparsed, expected);
 }
 
+TEST(parseFlakeRef, malformedGithubUrlDoesNotCrash)
+{
+    experimentalFeatureSettings.experimentalFeatures.get().insert(Xp::Flakes);
+
+    fetchers::Settings fetchSettings;
+
+    // Using ref= instead of rev= with a github: URL should produce an
+    // error, not an assertion failure in renderAuthorityAndPath
+    // (https://github.com/NixOS/nix/issues/15196).
+    EXPECT_THROW(
+        parseFlakeRef(fetchSettings, "github:nixos/nixpkgs/nixpkgs.git?ref=aead170c1a49253ebfa5027010dfd89a77b73ca4"),
+        Error);
+}
+
 } // namespace nix

src/libflake/flakeref.cc

@@ -205,7 +205,7 @@ std::pair<FlakeRef, std::string> parsePathFlakeRefWithFragment(
         fetchSettings,
         {
             .scheme = "path",
-            .authority = ParsedURL::Authority{},
+            .authority = isAbsolute(path) ? std::optional{ParsedURL::Authority{}} : std::nullopt,
             .path = splitString<std::vector<std::string>>(path, "/"),
             .query = query,
             .fragment = fragment,