Fix set of issues with extended statistics on expressions

Fix set of issues with extended statistics on expressions

This commit addresses two defects regarding extended statistics on
expressions:
- When building extended statistics in lookup_var_attr_stats(), the call
to examine_attribute() did not account for the possibility of a NULL
return value.  This can happen depending on the behavior of a typanalyze
callback — for example, if the callback returns false, if no rows are
sampled, or if no statistics are computed.  In such cases, the code
attempted to build MCV, dependency, and ndistinct statistics using a
NULL pointer, incorrectly assuming valid statistics were available,
which could lead to a server crash.
- When loading extended statistics for expressions,
statext_expressions_load() did not account for NULL entries in the
pg_statistic array storing expression statistics.  Such NULL entries can
be generated when statistics collection fails for an expression, as may
occur during the final step of serialize_expr_stats().  An extended
statistics object defining N expressions requires N corresponding
elements in the pg_statistic array stored for the expressions, and some
of these elements can be NULL.  This situation is reachable when a
typanalyze callback returns true, but sets stats_valid to indicate that
no useful statistics could be computed.

While these scenarios cannot occur with in-core typanalyze callbacks, as
far as I have analyzed, they can be triggered by custom data types with
custom typanalyze implementations, at least.

No tests are added in this commit.  A follow-up commit will introduce a
test module that can be extended to cover similar edge cases if
additional issues are discovered.  This takes care of the core of the
problem.

Attribute and relation statistics already offer similar protections:
- ANALYZE detects and skips the build of invalid statistics.
- Invalid catalog data is handled defensively when loading statistics.

This issue exists since the support for extended statistics on
expressions has been added, down to v14 as of a4d75c86bf15.  Backpatch
to all supported stable branches.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aaDrJsE1I5mrE-QF@paquier.xyz
Backpatch-through: 14