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());
 }