There was two problems:
The first was the symptom, caused by bad error handling in
ha_partition. It did not handle print_error etc. when
having no partitions (when used by dummy handler).

The second was the real problem that when dropping tables
it reused the table type (storage engine) from when the lock
was asked for, not the table type that it had when gaining
the exclusive name lock. So that it tried to delete tables
from wrong storage engines.

Solutions for the first problem was to accept some handler
calls to the partitioning handler even if it was not setup
with any partitions, and also if possible fallback
to use the base handler's default functions.

Solution for the second problem was to remove the optimization
to reuse the definition from the cache, instead always check
the frm-file when holding the LOCK_open mutex

(updated with a fix for a debug print crash and better
comments as required by reviewer, and removed optimization
to avoid reading the frm-file).