When an incremental join cache is used to join a table whose
fields are not referenced anywhere in the query the association
pointer to the last record in the such cache can be the same
as the pointer to the end of the buffer. 
The function JOIN_CACHE_BKA::get_next_key must take into 
consideration this when iterating over the keys of the records
from the join buffer. 
The assertion in JOIN_TAB_SCAN_MRR::next also must take this
into consideration.
Borrowed a slightly changed test case from a patch attached to the
bug #52394.