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ha_heap::external_lock contains some consistency checks for the table,#
in a debug compilation.

This code suffers from lack of synchronization, in a rare case
where mysql_lock_tables() fail, and unlock is forced, even if lock was
not previously taken.

To workaround, require EXTRA_DEBUG compile definition in order to activate
the consistency checks.The code still might be useful in some cases - but
the audience are developers looking for errors in single-threaded scenarios,
rather than multiuser stress-tests.