MDEV-34449 Default ha_spider::bulk_size to 0.

This fixes a valgrind failure where the bulk_size is used before
initialised in ha_spider::end_bulk_insert().

storage/spider/ha_spider.h

@@ -151,7 +151,7 @@ class ha_spider final : public handler
 #ifdef HANDLER_HAS_CAN_USE_FOR_AUTO_INC_INIT
   bool               auto_inc_temporary;
 #endif
-  int                bulk_size;
+  int                bulk_size= 0;
   int                direct_dup_insert;
   int                store_error_num;
   uint               dup_key_idx;