Repairing MyISAM table with fulltext indexes and low
myisam_sort_buffer_size may crash the server.

Estimation of number of index entries was done incorrectly,
causing further assertion failure or server crash.

Docs note: min value for myisam_sort_buffer_size has been
changed from 4 to 4096.

mysql-test/r/fulltext.result:
  A test case for BUG#51866.
mysql-test/r/myisam.result:
  Min value for myisam_sort_buffer_size is 4096.
mysql-test/r/variables.result:
  Min value for myisam_sort_buffer_size is 4096.
mysql-test/suite/sys_vars/r/myisam_sort_buffer_size_basic_32.result:
  Min value for myisam_sort_buffer_size is 4096.
mysql-test/t/fulltext.test:
  A test case for BUG#51866.
sql/mysqld.cc:
  Min value for myisam_sort_buffer_size is 4096.
storage/myisam/mi_check.c:
  When estimating number of index entries for external
  fulltext parser, take into account that key_length may
  be bigger than myisam_sort_buffer_size. Reuse logic
  from _create_index_by_sort(): force MIN_SORT_BUFFER to
  be min value for myisam_sort_buffer_size.
  
  Another problem is that ftkey_nr has no other meaning
  than serial number of fulltext index starting with 1.
  We can't say if this key using built-in or external
  parser basing on it's value. In other words we always
  entered if-branch for external parser. At this point,
  the only way to check if we use default parser is to
  compare keyinfo::parser with &ft_default_parser.
storage/myisam/sort.c:
  Get rid of MIN_SORT_MEMORY, use MIN_SORT_BUFFER instead
  (defined in myisamdef.h, has the same value and purpose).