Repair table could crash a server if there is not sufficient
memory (myisam_sort_buffer_size) to operate. Affects not only
repair, but also all statements that use create index by sort:
repair by sort, parallel repair, bulk insert.

Return an error if there is not sufficient memory to store at
least one key per BUFFPEK.

Also fixed memory leak if thr_find_all_keys returns an error.


myisam/sort.c:
  maxbuffer is number of BUFFPEK-s for repair. It is calculated
  as records / keys. keys is number of keys that can be stored
  in memory (myisam_sort_buffer_size). There must be sufficient
  memory to store both BUFFPEK-s and keys. It was checked
  correctly before this patch. However there is another
  requirement that wasn't checked: there must be sufficient
  memory for at least one key per BUFFPEK, otherwise repair
  by sort/parallel repair cannot operate.
  
  Return an error if there is not sufficient memory to store at
  least one key per BUFFPEK.
  
  Also fixed memory leak if thr_find_all_keys returns an error.
mysql-test/r/repair.result:
  A test case for BUG#23175.
mysql-test/t/repair.test:
  A test case for BUG#23175.