MDEV-30435 MDEV-30981 Fix ubsan errors w.r.t. memcpy in spd_trx.cc

Extract the indexed string memcopy pattern in spd_trx.cc to a static
inline function.

Also updated the ubsan check in mdev_26541.test (h/t roel).

storage/spider/mysql-test/spider/bugfix/r/mdev_30981.result

+#
+# MDEV-30981 Spider UBSAN: null pointer passed as argument 2, which is declared to never be null in spider_create_trx_alter_table on ALTER
+#
+for master_1
+for child2
+for child3
+CREATE TABLE t (c INT) ENGINE=Spider PARTITION BY LIST (c) (PARTITION p VALUES IN (1,2));
+ALTER TABLE t ENGINE=InnoDB;
+drop table t;
+for master_1
+for child2
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_26541.test

@@ -2,7 +2,7 @@
 --echo # MDEV-26541 Undefined symbol: _ZTI12ha_partition when attempting to use ha_spider.so in UBSAN builds
 --echo #
 
-if (`select not(count(*)) from information_schema.system_variables where variable_name='have_sanitizer' and global_value="UBSAN"`)
+if (`select not(count(*)) from information_schema.system_variables where variable_name='have_sanitizer' and global_value like "%UBSAN%"`)
 {
 --skip test needs to be run with UBSAN
 }

storage/spider/mysql-test/spider/bugfix/t/mdev_30981.test

+--echo #
+--echo # MDEV-30981 Spider UBSAN: null pointer passed as argument 2, which is declared to never be null in spider_create_trx_alter_table on ALTER
+--echo #
+
+if (`select not(count(*)) from information_schema.system_variables where variable_name='have_sanitizer' and global_value like "%UBSAN%"`)
+{
+--skip test needs to be run with UBSAN
+}
+ 
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+ 
+CREATE TABLE t (c INT) ENGINE=Spider PARTITION BY LIST (c) (PARTITION p VALUES IN (1,2));
+ALTER TABLE t ENGINE=InnoDB;
+drop table t;
+ 
+--disable_query_log
+--disable_result_log
+--source ../t/test_deinit.inc
+--enable_query_log
+--enable_result_log