The bug was due to a loss happened during a refactoring made
on May 30 2005 that modified the function JOIN::reinit.
As a result of it for any subquery the value of offset_limit_cnt
was not restored for the following executions. Yet the first 
execution of the subquery made it equal to 0.
The fix restores this value in the function JOIN::reinit.  


mysql-test/r/subselect.result:
  Added a test case fir bug #20519.
mysql-test/t/subselect.test:
  Added a test case fir bug #20519.