Problem was that JOIN_TAB::cleanup() was not run because
JOIN::top_join_tab_count was not set in case of early errors.

Fixed by setting JOIN::tab_join_tab_count when JOIN_TAB's are allocated.

Something that should eventually be fixed:
- Cleaning up JOIN_TAB's is now done in 3 different loops.
  JOIN_TAB::cleanup() is only doing a partial cleanup. Other cleanups
  are done outside of JOIN_TAB::cleanup().

The above should be fixed so that JOIN_TAB::cleanup() is freeing
everything related to it's own memory, including all its sub JOIN_ TAB's.
JOIN::cleanup() should only loop over all it's top JOIN_TAB's and call
JOIN_TAB::cleanup() on these.
This will greatly simplify and speedup the current code (as we now do some
cleanup's twice).