We disallow the partitioning of a log table. You could however
partition a table first, and then point logging to it. This is
not only against the docs, it also crashes the server.

We catch this case now.

mysql-test/r/partition.result:
  results for 40281
mysql-test/t/partition.test:
  test for 40281: show that trying to log to partitioned table fails rather
  to crash the server
sql/ha_partition.cc:
  Signal that we no longer support logging to partitioned tables,
  as per the docs.
sql/sql_partition.cc:
  Some commands like "USE ..." have no select, yet we may try
  to parse partition info after their execution if user set a
  partitioned table as log target. This shouldn't lead to a
  NULL-deref/crash.