This commit updates the path matching logic in getServeHandler for malformed
request targets like "*" (e.g. "GET *") and "" (e.g. "CONNECT" authority-form).
Those paths never reduce to "/" as absolute path would. An absolute path check
was added and an additional check on no further reduce was added in the loop.
Fixestailscale/corp#44814
Signed-off-by: kevinliang10 <kevinliang@tailscale.com>