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:
  Bug #43354: test case
mysql-test/t/explain.test:
  Bug #43354: test case
sql/item_sum.cc:
  Bug #43354: use the argument list copy only if it's initialized