Write operations on tables created in 4.x with index on variable
length column results in index crash. Even REPAIR TABLE wasn't able
to fix broken index.

Problem was that packed key length size wasn't restored correctly.
In 5.0 packed key length size is either 1 or 2. In 4.x this length
is always 2, but is saved as 0.

This fix ensures that key length size is restored correctly for 4.x
tables.