Fix for BUG#6825: When calculating Item_func_neg::max_length, add 1 for '-'.

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

mysql-test/r/func_concat.result

@@ -32,3 +32,39 @@ select * from t1 where concat(A,C,B,D) = 'AAAA2003-03-011051';
 a	b	c	d
 AAAA	105	2003-03-01	1
 drop table t1;
+select 'a' union select concat('a', -4);
+a
+a
+a-4
+select 'a' union select concat('a', -4.5);
+a
+a
+a-4.5
+select 'a' union select concat('a', -(4 + 1));
+a
+a
+a-5
+select 'a' union select concat('a', 4 - 5);
+a
+a
+a-1
+select 'a' union select concat('a', -'3');
+a
+a
+a-3
+select 'a' union select concat('a', -concat('3',4));
+a
+a
+a-34
+select 'a' union select concat('a', -0);
+a
+a
+a0
+select 'a' union select concat('a', -0.0);
+a
+a
+a-0.0
+select 'a' union select concat('a', -0.0000);
+a
+a
+a-0.0000