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

heap/hp_create.c

@@ -41,6 +41,13 @@ int heap_create(const char *name, uint keys, HP_KEYDEF *keydef,
   {
     HP_KEYDEF *keyinfo;
     DBUG_PRINT("info",("Initializing new table"));
+    
+    /*
+      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, keyinfo= keydef; i < keys; i++, keyinfo++)
     {
       bzero((char*) &keyinfo->block,sizeof(keyinfo->block));

mysql-test/r/heap.result

@@ -240,3 +240,12 @@ SELECT * FROM t1;
 pseudo	date
 ZoomZip	1101106546
 DROP TABLE t1;
+create table t1(a char(2)) engine=memory;
+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

@@ -185,3 +185,13 @@ DELETE FROM t1 WHERE date<1101106546;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+#
+# Bug #6878: a problem with small length records
+#
+
+create table t1(a char(2)) engine=memory;
+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;