MDEV-17830 Server crashes in Item_null_result::field_type upon SELECT with CHARSET(date) and ROLLUP

mysql-test/r/olap.result

@@ -786,5 +786,19 @@ t	COUNT(*)
 12:12:13	1
 DROP TABLE t1;
 #
+# MDEV-17830 Server crashes in Item_null_result::field_type upon SELECT with CHARSET(date) and ROLLUP
+#
+# Note, returning "latin1" in the first row vs "binary" in the second row is wrong here.
+# Both lines should return equal values.
+# The point in this test is to make sure it does not crash.
+# Bad result will be fixed in a later version.
+CREATE TABLE t (d DATE) ENGINE=MyISAM;
+INSERT INTO t VALUES ('2018-12-12');
+SELECT CHARSET(d) AS f FROM t GROUP BY d WITH ROLLUP;
+f
+latin1
+binary
+DROP TABLE t;
+#
 # End of 10.1 tests
 #

mysql-test/t/olap.test

@@ -424,6 +424,21 @@ SELECT t, COUNT(*) FROM t1 GROUP BY t WITH ROLLUP HAVING t > '00:00:00';
 DROP TABLE t1;
 
 
+--echo #
+--echo # MDEV-17830 Server crashes in Item_null_result::field_type upon SELECT with CHARSET(date) and ROLLUP
+--echo #
+
+--echo # Note, returning "latin1" in the first row vs "binary" in the second row is wrong here.
+--echo # Both lines should return equal values.
+--echo # The point in this test is to make sure it does not crash.
+--echo # Bad result will be fixed in a later version.
+
+CREATE TABLE t (d DATE) ENGINE=MyISAM;
+INSERT INTO t VALUES ('2018-12-12');
+SELECT CHARSET(d) AS f FROM t GROUP BY d WITH ROLLUP;
+DROP TABLE t;
+
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/item.h

@@ -2618,6 +2618,10 @@ class Item_null_result :public Item_null
   {
     return result_field->type();
   }
+  CHARSET_INFO *charset_for_protocol(void) const
+  {
+    return collation.collation;
+  }
 #else
   const Type_handler *type_handler() const
   {
@@ -3942,6 +3946,12 @@ class Item_ref :public Item_ident
   void save_org_in_field(Field *field, fast_field_copier optimizer_data);
   fast_field_copier setup_fast_field_copier(Field *field)
   { return (*ref)->setup_fast_field_copier(field); }
+#if MARIADB_VERSION_ID < 100300
+  CHARSET_INFO *charset_for_protocol(void) const
+  {
+    return (*ref)->charset_for_protocol();
+  }
+#endif
   enum Item_result result_type () const { return (*ref)->result_type(); }
   enum_field_types field_type() const   { return (*ref)->field_type(); }
   Field *get_tmp_table_field()