DB-771 fix incorrect assert in update callback for varchar expansion

mysql-test/suite/tokudb.bugs/r/db771.result

+set default_storage_engine=tokudb;
+drop table if exists t1;
+CREATE TABLE t1 (a int key, b varchar(32), c varchar(32));
+REPLACE t1 SET a = 4;
+ALTER TABLE t1 CHANGE COLUMN c c VARCHAR(500);
+update t1 set b='hi';
+update t1 set c='there';
+select * from t1;
+a	b	c
+4	hi	there
+drop table t1;

mysql-test/suite/tokudb.bugs/t/db771.test

+# test case for DB-771
+source include/have_tokudb.inc;
+set default_storage_engine=tokudb;
+disable_warnings;
+drop table if exists t1;
+enable_warnings;
+CREATE TABLE t1 (a int key, b varchar(32), c varchar(32));
+REPLACE t1 SET a = 4;
+ALTER TABLE t1 CHANGE COLUMN c c VARCHAR(500);
+update t1 set b='hi';
+update t1 set c='there';
+select * from t1;
+drop table t1;

mysql-test/suite/tokudb/r/change_column_varchar.result

@@ -6,6 +6,7 @@ Table	Create Table
 t	CREATE TABLE `t` (
   `a` varchar(1) DEFAULT NULL
 ) ENGINE=TokuDB DEFAULT CHARSET=latin1
+INSERT INTO t VALUES (null);
 ALTER TABLE t CHANGE COLUMN a a VARCHAR(2);
 ALTER TABLE t CHANGE COLUMN a a VARCHAR(2);
 ALTER TABLE t CHANGE COLUMN a a VARCHAR(3);

mysql-test/suite/tokudb/t/change_column_varchar.test

@@ -9,6 +9,7 @@ SET SESSION TOKUDB_DISABLE_SLOW_ALTER=ON;
 
 CREATE TABLE t (a VARCHAR(1)) ENGINE=TokuDB;
 SHOW CREATE TABLE t;
+INSERT INTO t VALUES (null);
 
 # 1->1
 let $i=1

storage/tokudb/tokudb_update_fun.cc

@@ -851,7 +851,7 @@ static int tokudb_expand_variable_offsets(
     DBT new_val; memset(&new_val, 0, sizeof new_val);
 
     if (old_val != NULL) {
-        assert(offset_start + number_of_offsets < old_val->size);
+        assert(offset_start + number_of_offsets <= old_val->size);
     
         // compute the new val from the old val
         uchar *old_val_ptr = (uchar *)old_val->data;