MDEV-20855 Crash with PARTITION BY LIST and extended characters

The code in convert_charset_partition_constant() did not
take into account that the call for item->safe_charset_converter()
can return NULL when conversion is not safe.

Note, 10.2 was not affected. The test for NULL presents in 10.2,
but it disappeared in 10.3 in a mistake. Restoring the test.

mysql-test/main/partition_ucs2.result

+#
+# Start of 10.3 tests
+#
+#
+# MDEV-20855 Crash with PARTITION BY LIST and extended characters
+#
+SET NAMES utf8;
+CREATE OR REPLACE TABLE t1 (a TIME)
+PARTITION BY LIST COLUMNS (a) (PARTITION p0 VALUES IN (_ucs2 0x0411));
+ERROR HY000: This partition function is not allowed
+#
+# End of 10.3 tests
+#

mysql-test/main/partition_ucs2.test

+--source include/have_partition.inc
+--source include/have_ucs2.inc
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-20855 Crash with PARTITION BY LIST and extended characters
+--echo #
+
+SET NAMES utf8;
+--error ER_PARTITION_FUNCTION_IS_NOT_ALLOWED
+CREATE OR REPLACE TABLE t1 (a TIME)
+  PARTITION BY LIST COLUMNS (a) (PARTITION p0 VALUES IN (_ucs2 0x0411));
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

mysql-test/main/partition_utf8.result

@@ -51,3 +51,19 @@ t1	CREATE TABLE `t1` (
 insert into t1 values ('');
 insert into t1 values (_ucs2 0x2020);
 drop table t1;
+#
+# Start of 10.3 tests
+#
+#
+# MDEV-20855 Crash with PARTITION BY LIST and extended characters
+#
+SET NAMES utf8;
+CREATE OR REPLACE TABLE t1 (a CHAR(10)) CHARACTER SET latin1
+PARTITION BY LIST COLUMNS (a) (PARTITION p0 VALUES IN ('Б'));
+ERROR HY000: This partition function is not allowed
+CREATE OR REPLACE TABLE t1 (a TIME)
+PARTITION BY LIST COLUMNS (a) (PARTITION p0 VALUES IN ('Б'));
+ERROR HY000: This partition function is not allowed
+#
+# End of 10.3 tests
+#

mysql-test/main/partition_utf8.test

@@ -40,3 +40,25 @@ show create table t1;
 insert into t1 values ('');
 insert into t1 values (_ucs2 0x2020);
 drop table t1;
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-20855 Crash with PARTITION BY LIST and extended characters
+--echo #
+
+SET NAMES utf8;
+
+--error ER_PARTITION_FUNCTION_IS_NOT_ALLOWED
+CREATE OR REPLACE TABLE t1 (a CHAR(10)) CHARACTER SET latin1
+  PARTITION BY LIST COLUMNS (a) (PARTITION p0 VALUES IN ('Б'));
+
+--error ER_PARTITION_FUNCTION_IS_NOT_ALLOWED
+CREATE OR REPLACE TABLE t1 (a TIME)
+  PARTITION BY LIST COLUMNS (a) (PARTITION p0 VALUES IN ('Б'));
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/sql_partition.cc

@@ -146,7 +146,7 @@ Item* convert_charset_partition_constant(Item *item, CHARSET_INFO *cs)
   item= item->safe_charset_converter(thd, cs);
   context->table_list= NULL;
   thd->where= "convert character set partition constant";
-  if (item->fix_fields_if_needed(thd, (Item**)NULL))
+  if (item && item->fix_fields_if_needed(thd, (Item**)NULL))
     item= NULL;
   thd->where= save_where;
   context->table_list= save_list;