checked for each record'

The problem was in incorrectly calculated length of the buffer used to
store a hexadecimal representation of an index map in
select_describe(). This could result in buffer overrun and stack
corruption under some circumstances.

Fixed by correcting the calculation.


mysql-test/r/explain.result:
  Added a test case for bug #32241.
mysql-test/t/explain.test:
  Added a test case for bug #32241.
sql/sql_select.cc:
  Corrected the buffer length calculation. Count one hex digit as 4 bits,
  not 8.