returns short string value. 
 
Multibyte character sets were not taken into account when 
calculating max_length in Item_param::convert_str_value(). As a 
result, string parameters of a prepared statement could be 
truncated later when calculating string length in characters by 
dividing length in bytes by the charset's mbmaxlen value (e.g. in 
Field_varstring::store()). 
 
Fixed by taking charset's mbmaxlen into account when calculating 
max_length in Item_param::convert_str_value().