Bug #43354: Use key hint can crash server in explain extended query

The copy of the original arguments of a aggregate function was not
initialized until after fix_fields().
Sometimes (e.g. when there's an error processing the statement)
the print() can be called with no corresponding fix_fields() call.
      
Fixed by adding a check if the Item is fixed before using the arguments
copy.

mysql-test/r/explain.result

@@ -155,3 +155,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 Warnings:
 Note	1003	select 1 AS `1` from (select count(distinct `test`.`t1`.`a`) AS `COUNT(DISTINCT t1.a)` from `test`.`t1` join `test`.`t2` group by `test`.`t1`.`a`) `s1`
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a INT PRIMARY KEY);
+EXPLAIN EXTENDED SELECT COUNT(a) FROM t1 USE KEY(a);
+ERROR HY000: Key 'a' doesn't exist in table 't1'
+DROP TABLE t1;