(also 5.5+ solution for bug#11766879/bug#60106)

The valgrind warning was due to an unused 'new handler_add_index(...)'
which was never freed.

The error handling did not work (fails as in bug#11766879) and
the implementation was not as transparant as it could, therefore I
made it a bit simpler and more transparant to the underlying handlers.

This way it follows the api better and the error handling works and
is also now tested.

Also added a debug test to verify the error handling.

Improved according to Jon Olavs review:
Added class ha_partition_add_index.
Also added base class Sql_alloc to handler_add_index.
Update 3.