MDEV-4813 Replication fails on updating a MEMORY table with an index using btree

skip NULL VARCHAR key parts like it's done elsewhere

mysql-test/suite/heap/btree_varchar_null.result

+create table t1 (f1 varchar(128), f2 varchar(128), key (f2,f1) using btree) engine=memory;
+insert into t1 values (null,'not'),('one',null),('two',null),('three','');
+select * from t1 where f1 = 'one'  and f2 is null;
+f1	f2
+one	NULL
+drop table t1;

mysql-test/suite/heap/btree_varchar_null.test

+#
+# MDEV-4813 Replication fails on updating a MEMORY table with an index using btree
+#
+create table t1 (f1 varchar(128), f2 varchar(128), key (f2,f1) using btree) engine=memory;
+insert into t1 values (null,'not'),('one',null),('two',null),('three','');
+select * from t1 where f1 = 'one'  and f2 is null;
+drop table t1;

storage/heap/hp_hash.c

@@ -876,8 +876,13 @@ uint hp_rb_pack_key(HP_KEYDEF *keydef, uchar *key, const uchar *old,
     if (seg->null_bit)
     {
       if (!(*key++= (char) 1 - *old++))
+      {
+	/* Add key pack length (2) to key for VARCHAR segments */
+        if (seg->type == HA_KEYTYPE_VARTEXT1)
+          old+= 2;
         continue;
       }
+    }
     if (seg->flag & HA_SWAP_KEY)
     {
       uint length= seg->length;