A corrupt table with dynamic record format can crash the 
server when trying to select from it.
        
I fixed the crash that resulted from the particular type 
of corruption that has been reported for this bug.

No test case. To test it, one needs a table with a very special
corruption. The bug report contains a file with such a table.


myisam/mi_dynrec.c:
  Bug#19835 - Binary copy of corrupted tables crash the server when issuing a query
  Added a protection against corrupted records. A dynamic 
  record header with invalid 'next' pointer could trigger 
  the assert in _mi_get_block_info(). Now I avoid this by 
  reporting a corruption error.