Fix bogus code for extracting extended-statistics data from syscache.

statext_dependencies_load and statext_ndistinct_load were not up to snuff,
in addition to being randomly different from each other.  In detail:

* Deserialize the fetched bytea value before releasing the syscache
entry, not after.  This mistake causes visible regression test failures
when running with -DCATCACHE_FORCE_RELEASE.  Since it's not exposed by
-DCLOBBER_CACHE_ALWAYS, I think there may be no production hazard here
at present, but it's at least a latent bug.

* Use DatumGetByteaPP not DatumGetByteaP to save a detoasting cycle
for short stats values; the deserialize function has to be, and is,
prepared for short-header values since its other caller uses PP.

* Use a test-and-elog for null stats values in both functions, rather
than a test-and-elog in one case and an Assert in the other.  Perhaps
Asserts would be sufficient in both cases, but I don't see a good
argument for them being different.

* Minor cosmetic changes to make these functions more visibly alike.

Backpatch to v10 where this code came in.

Amit Langote, minor additional hacking by me

Discussion: https://postgr.es/m/1349aabb-3a1f-6675-9fc0-65e2ce7491dd@lab.ntt.co.jp

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