For numeric constants we only need to add, since the parser doesn't produce 
negative numbers. 
For strings we only add (we actually could substract 1 if given string is a constant 
and it has '-number' form but we're not doing that because 
 * we set max_length bigger then necessary in other cases as well.  
 * the current solution is simpler and safer (bigger max_length is better then cutting out)


mysql-test/r/func_concat.result:
  Test for BUG#6825
mysql-test/r/metadata.result:
  Ajusted results according to fix of bug BUG#6825:length(-1) = 2 , not 1
mysql-test/t/func_concat.test:
  Test for BUG#6825