radix tree test suite: Fix leaky tests

If item_insert() or item_insert_order() failed to insert an item, they
would leak the item they had just created.  This was causing runaway
memory consumption while running the iteration_check testcase, which
proves that Ross has too much memory in his workstation ;-)

Make sure to free the item on error.  Found with -fsanitize=address.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Rehas Sachdeva <aquannie@gmail.com>