MDEV-34002 Initialise fields in spider_db_handler

Otherwise it may result in nonsensical values like 190 for a boolean.

storage/spider/mysql-test/spider/bugfix/r/mdev_34002.result

+INSTALL PLUGIN Spider SONAME 'ha_spider.so';
+CREATE TABLE t (c DATE, c2 VARCHAR(1025) CHARACTER SET utf8mb3, UNIQUE KEY k(c2)) ENGINE=SPIDER;
+UPDATE t SET c='２';
+ERROR HY000: Unable to connect to foreign data source: localhost
+drop table t;
+Warnings:
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+Note	1305	PLUGIN SPIDER_ALLOC_MEM does not exist
+Note	1305	PLUGIN SPIDER_WRAPPER_PROTOCOLS does not exist

storage/spider/mysql-test/spider/bugfix/t/mdev_34002.test

+INSTALL PLUGIN Spider SONAME 'ha_spider.so';
+CREATE TABLE t (c DATE, c2 VARCHAR(1025) CHARACTER SET utf8mb3, UNIQUE KEY k(c2)) ENGINE=SPIDER;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+UPDATE t SET c='２';
+drop table t;
+--disable_query_log
+--source ../../include/clean_up_spider.inc

storage/spider/spd_db_include.h

@@ -1136,11 +1136,8 @@ class spider_db_handler
   ha_spider *spider;
   spider_db_share *db_share;
   int first_link_idx;
-#ifdef SPIDER_HAS_GROUP_BY_HANDLER
-  SPIDER_LINK_IDX_CHAIN *link_idx_chain;
-#endif
-  bool strict_group_by;
-  bool no_where_cond;
+  bool strict_group_by= false;
+  bool no_where_cond= false;
   spider_db_handler(ha_spider *spider, spider_db_share *db_share) :
     dbton_id(db_share->dbton_id), spider(spider), db_share(db_share),
     first_link_idx(-1) {}