IF(..., CAST(longtext AS UNSIGNED), signed_val)
            (was: LEFT JOIN on inline view crashes server)

Select from a LONGTEXT column wrapped with an expression
like "IF(..., CAST(longtext_column AS UNSIGNED), smth_signed)"
failed an assertion or crashed the server. IFNULL function was
affected too.

LONGTEXT column item has a maximum length of 32^2-1 bytes,
at the same time this is a maximum possible length of any
MySQL item. CAST(longtext_column AS UNSIGNED) returns some
unsigned numeric result of length 32^2-1, so the result of
IF/IFNULL function of this number and some other signed number
will have text length of (32^2-1)+1=32^2 (one byte for the
minus sign) - there is integer overflow, and the length is
equal to zero. That caused assert/crash.

The bug has been fixed by the same solution as in the CASE
function implementation.


mysql-test/r/func_if.result:
  Added test case for bug #40761.
mysql-test/t/func_if.test:
  Added test case for bug #40761.
sql/item_cmpfunc.cc:
  Bug #40761: Assert on sum function on
              IF(..., CAST(longtext AS UNSIGNED), signed_val)
  
  1. Item_func_case::agg_str_lengths method has been moved
     to the Item_func superclass.
  2. Item_func_ifnull/Item_func_if::fix_length_and_dec methods
     have been updated to calculate max_length, decimals and
     unsigned flag like Item_func_case.
sql/item_cmpfunc.h:
  Bug #40761: Assert on sum function on
              IF(..., CAST(longtext AS UNSIGNED), signed_val)
  
  Item_func_case::agg_str_lengths method has been moved to
  the Item_func superclass.
sql/item_func.cc:
  Bug #40761: Assert on sum function on
              IF(..., CAST(longtext AS UNSIGNED), signed_val)
  
  Item_func_case::agg_str_lengths method has been moved to
  the Item_func superclass.
sql/item_func.h:
  Bug #40761: Assert on sum function on
              IF(..., CAST(longtext AS UNSIGNED), signed_val)
  
  Item_func_case::agg_str_lengths method has been moved to
  the Item_func superclass.