A fix (bug #6878: Crash with engine=memory).

heap/hp_open.c

@@ -41,6 +41,13 @@ HP_INFO *heap_open(const char *name, int mode, uint keys, HP_KEYDEF *keydef,
   {
     DBUG_PRINT("info",("Initializing new table"));
     implicit_emptied= 1;
+
+    /*
+      We have to store sometimes byte* del_link in records,
+      so the record length should be at least sizeof(byte*)
+    */
+    set_if_bigger(reclength, sizeof (byte*));
+
     for (i=key_segs=max_length=0 ; i < keys ; i++)
     {
       key_segs+= keydef[i].keysegs;

mysql-test/r/heap.result

@@ -227,3 +227,12 @@ SELECT MAX(job_title_id) FROM job_titles;
 MAX(job_title_id)
 NULL
 DROP TABLE job_titles;
+create table t1(a char(2)) type=heap;
+insert into t1 values (NULL), (NULL);
+delete from t1 where a is null;
+insert into t1 values ('2'), ('3');
+select * from t1;
+a
+3
+2
+drop table t1;

mysql-test/t/heap.test

@@ -161,3 +161,14 @@ CREATE TABLE `job_titles` (
 SELECT MAX(job_title_id) FROM job_titles;
 
 DROP TABLE job_titles;
+
+#
+# Bug #6878: a problem with small length records
+#
+
+create table t1(a char(2)) type=heap;
+insert into t1 values (NULL), (NULL);
+delete from t1 where a is null;
+insert into t1 values ('2'), ('3');
+select * from t1;
+drop table t1;