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().