MDEV-34404 Use safe_str in spider udfs to avoid passing NULL str

sql/sql_class.h

@@ -4580,7 +4580,7 @@ class THD: public THD_count, /* this must be first */
     to->length= db.length;
     return to->str == NULL;                     /* True on error */
   }
-  /* Get db name or "". Use for printing current db */
+  /* Get db name or "". */
   const char *get_db()
   { return safe_str(db.str); }
 

storage/spider/spd_copy_tables.cc

@@ -52,7 +52,7 @@ int spider_udf_set_copy_tables_param_default(
     copy_tables->database_length = SPIDER_THD_db_length(copy_tables->trx->thd);
     if (
       !(copy_tables->database = spider_create_string(
-        SPIDER_THD_db_str(copy_tables->trx->thd),
+        copy_tables->trx->thd->get_db(),
         copy_tables->database_length))
     ) {
       my_error(ER_OUT_OF_RESOURCES, MYF(0), HA_ERR_OUT_OF_MEM);

storage/spider/spd_direct_sql.cc

@@ -1109,7 +1109,7 @@ int spider_udf_set_direct_sql_param_default(
     direct_sql->tgt_default_db_name_length = SPIDER_THD_db_length(trx->thd);
     if (
       !(direct_sql->tgt_default_db_name = spider_create_string(
-        SPIDER_THD_db_str(trx->thd),
+          trx->thd->get_db(),
         direct_sql->tgt_default_db_name_length))
     ) {
       my_error(ER_OUT_OF_RESOURCES, MYF(0), HA_ERR_OUT_OF_MEM);