Fix invalid iterator bump in ValueSplittingHeaderListTest This was found with _LIBCPP_DEBUG=1. ValueSplittingHeaderListTest tests 6 iterator values, the last of which is headers.end(). That means, at the last iterator, it cannot run it++. PiperOrigin-RevId: 279095545 Change-Id: Id59dcb22faee4da4fdd8a4fac080418aa53e8caf