MDEV-39356: Server crashes when executing UPDATE ... FOR PORTION OF with a normal table

Problem:
  Executing an `UPDATE ... FOR PORTION OF` statement on a table without
  a defined period results in assertion failure. The code in
  `SELECT_LEX::period_setup_conds` attempts to compare a NULL
  period name identifier using `streq()`, which triggers the assertion
  in `strnncoll` funtion.

Fix:
  Replace `streq` with `streq_safe` in `SELECT_LEX::period_setup_conds`
  to handle NULL pointers safely via an early-exit.

Author: raghunandanbhat

mysql-test/main/ctype_utf8mb3_geeral1400_as_ci.result

@@ -29,4 +29,11 @@ DROP TABLE t1;
 CREATE TABLE t (c INT,c2 CHAR,c3 DATE,CHECK (c>0));
 ALTER TABLE t ADD INDEX (c2) USING HASH;
 DROP TABLE t;
+#
+# MDEV-39356: Server crashes when executing UPDATE ... FOR PORTION OF with a normal table
+#
+CREATE OR REPLACE TABLE t (id INT);
+UPDATE t FOR PORTION OF p FROM '2026-02-01' TO '2026-03-01'  SET id = 2;
+ERROR HY000: Period `p` is not found in table
+DROP TABLE t;
 # End of 11.8 tests

mysql-test/main/ctype_utf8mb3_geeral1400_as_ci.test

@@ -33,4 +33,13 @@ ALTER TABLE t ADD INDEX (c2) USING HASH;
 
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-39356: Server crashes when executing UPDATE ... FOR PORTION OF with a normal table
+--echo #
+
+CREATE OR REPLACE TABLE t (id INT);
+--error ER_PERIOD_NOT_FOUND
+UPDATE t FOR PORTION OF p FROM '2026-02-01' TO '2026-03-01'  SET id = 2;
+DROP TABLE t;
+
 --echo # End of 11.8 tests

sql/sql_select.cc

@@ -1174,7 +1174,7 @@ int SELECT_LEX::period_setup_conds(THD *thd, TABLE_LIST *tables)
     if (!table->table)
       continue;
     vers_select_conds_t &conds= table->period_conditions;
-    if (!table->table->s->period.name.streq(conds.name))
+    if (!table->table->s->period.name.streq_safe(conds.name))
     {
       my_error(ER_PERIOD_NOT_FOUND, MYF(0), conds.name.str);
       if (arena)