Fixed a problem for Q18 from DBT3/SF30 with innodb database instance:

the server crashed when running the query with persistent statistics
enabled.
The field KEY::read_stat.avg_frequency must be initialized to NULL
for the keys of temporary tables.
 

sql/sql_select.cc

@@ -14657,6 +14657,7 @@ create_tmp_table(THD *thd, TMP_TABLE_PARAM *param, List<Item> &fields,
     keyinfo->algorithm= HA_KEY_ALG_UNDEF;
     keyinfo->is_statistics_from_stat_tables= FALSE;
     keyinfo->rec_per_key=0;
+    keyinfo->read_stat.avg_frequency= NULL;
 
     /*
       Create an extra field to hold NULL bits so that unique indexes on

sql/sql_statistics.cc

@@ -1636,6 +1636,7 @@ void set_statistics_for_table(THD *thd, TABLE *table)
        key_info < key_info_end; key_info++)
   {
     key_info->is_statistics_from_stat_tables=
-      (use_stat_table_mode > 1 && key_info->read_stat.avg_frequency[0] > 0.5);
+      (use_stat_table_mode > 1 &&  key_info->read_stat.avg_frequency &&
+       key_info->read_stat.avg_frequency[0] > 0.5);
   }
 }

sql/table.cc

@@ -5957,6 +5957,7 @@ bool TABLE::add_tmp_key(uint key, uint key_parts,
   if (!keyinfo->rec_per_key)
     return TRUE;
   bzero(keyinfo->rec_per_key, sizeof(ulong)*key_parts);
+  keyinfo->read_stat.avg_frequency= NULL;
 
   for (i= 0; i < key_parts; i++)
   {