JOIN, and ORDER BY

Problem: improper maximum length calculation of the CASE function leads to 
decimal value truncation (storing/retrieving decimal field values).

Fix: accurately calculate maximum length/unsigned flag/decimals parameters 
of the CASE function.


mysql-test/r/case.result:
  Fix for bug #30782: Truncated UNSIGNED BIGINT columns only in SELECT w/ CASE, 
  JOIN, and ORDER BY
    - test result.
mysql-test/t/case.test:
  Fix for bug #30782: Truncated UNSIGNED BIGINT columns only in SELECT w/ CASE, 
  JOIN, and ORDER BY
    - test case.
sql/item_cmpfunc.cc:
  Fix for bug #30782: Truncated UNSIGNED BIGINT columns only in SELECT w/ CASE, 
  JOIN, and ORDER BY
    - accurately calculate Item_func_case::max_length/unsigned_flag/decimals.
sql/item_cmpfunc.h:
  Fix for bug #30782: Truncated UNSIGNED BIGINT columns only in SELECT w/ CASE, 
  JOIN, and ORDER BY
    - accurately calculate Item_func_case::max_length/unsigned_flag/decimals.