list-objects-filter: correct usage of ALLOC_GROW

In the sparse filter data, array_frame array is used in a way such that
nr is the index of the last element. Fix this so that nr is actually the
number of elements in the array.

The filter_sparse_free function also has an unaddressed TODO to free the
memory associated with the sparse filter data. Address that TODO and fix
the memory leak.

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
