Bug #55568: user variable assignments crash server when used

within query

The server could crash after materializing a derived table
which requires a temporary table for grouping.

When destroying the temporary table used to execute a query for
a derived table, JOIN::destroy() did not clean up Item_fields
pointing to fields in the temporary table. This led to
dereferencing a dangling pointer when printing out the items
tree later in the outer SELECT.

The solution is an addendum to the patch for bug37362: in
addition to cleaning up items in tmp_all_fields3, do the same
for items in tmp_all_fields1, since now we have an example
where this is necessary.


mysql-test/r/join.result:
  Added test cases for bug#55568 and a duplicate bug #54468.
mysql-test/t/join.test:
  Added test cases for bug#55568 and a duplicate bug #54468.
sql/field.cc:
  Make sure field->table_name is not set to NULL in
  Field::make_field() to avoid assertion failure in 
  Item_field::make_field() after cleaning up items
  (the assertion fired in udf.test when running
  the test suite with the patch applied).
sql/sql_select.cc:
  In addition to cleaning up items in tmp_all_fields3, do the
  same for items in tmp_all_fields1.
  Introduce a new helper function to avoid code duplication.
sql/sql_select.h:
  Introduce a new helper function to avoid code duplication in
  JOIN::destroy().

sql/field.cc

@@ -1535,7 +1535,7 @@ void Field::make_field(Send_field *field)
   }
   else
     field->org_table_name= field->db_name= "";
-  if (orig_table)
+  if (orig_table && orig_table->alias)
   {
     field->table_name= orig_table->alias;
     field->org_col_name= field_name;