`DeserializeHDKeypaths()` was writing into the original `hd_keypaths`
map instead of `deserialized_hd_keypaths`. As a result the latter was
always empty and the round-trip assertion following was trivially true,
so the serialize/deserialize round-trip wasn't actually being exercised.
That bug was introduced with the commit introducing the fuzz target
(commit f898ef65c9, #18994).