Fix for bug#58553, "Queries with pushed conditions causes 'explain extended'

to crash mysqld". 
      
handler::pushed_cond was not always properly reset when table objects where
recycled via the table cache.
      
handler::pushed_cond is now set to NULL in handler::ha_reset(). This should 
prevent pushed conditions from (incorrectly) re-apperaring in later queries.

sql/handler.cc

@@ -4657,6 +4657,7 @@ int handler::ha_reset()
   free_io_cache(table);
   /* reset the bitmaps to point to defaults */
   table->default_column_bitmaps();
+  pushed_cond= NULL;
   DBUG_RETURN(reset());
 }
 

sql/sql_base.cc

@@ -2993,6 +2993,11 @@ TABLE *open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
   table->insert_values= 0;
   table->fulltext_searched= 0;
   table->file->ft_handler= 0;
+  /*
+    Check that there is no reference to a condtion from an earlier query
+    (cf. Bug#58553). 
+  */
+  DBUG_ASSERT(table->file->pushed_cond == NULL);
   table->reginfo.impossible_range= 0;
   /* Catch wrong handling of the auto_increment_field_not_null. */
   DBUG_ASSERT(!table->auto_increment_field_not_null);

sql/sql_select.cc

@@ -6380,7 +6380,6 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
           tab->select_cond=sel->cond=tmp;
           /* Push condition to storage engine if this is enabled
              and the condition is not guarded */
-          tab->table->file->pushed_cond= NULL;
 	  if (thd->variables.engine_condition_pushdown)
           {
             COND *push_cond=