To calculate its max_length the CONCAT() function is simply sums max_lengths
of its arguments but when the collation of an argument differs from the 
collation of the CONCAT() max_length will be wrong. This may lead to a data
truncation when a tmp table is used, in UNIONS for example.

The Item_func_concat::fix_length_and_dec() function now recalculates the 
max_length of an argument when the mbmaxlen of the argument differs from the
mbmaxlen of the CONCAT().


mysql-test/t/func_concat.test:
  Added test case for bug#15962:CONCAT() in UNION may lead to a data trucation.
mysql-test/r/func_concat.result:
  Added test case for bug#15962:CONCAT() in UNION may lead to a data trucation.
sql/item_strfunc.cc:
  Fixed bug#15962: CONCAT() in UNION may lead to a data trucation.
  The Item_func_concat::fix_length_and_dec() function now recalculates the 
  max_length of an argument when the mbmaxlen of the argument differs from the
  mbmaxlen of the CONCAT().