[fidl] Ensure handles are closed in all error cases.

The contract for fidl_encode and fidl_decode is that all handles must be
closed in case of an error. The big refactor to visitors silently broke
that contract in error cases where the caller supplied an overly large
buffer/handles array. This patch fixes the behavior and adds tests.

TEST: /boot/test/sys/fidl-test
Change-Id: I50e81f3c9168370f08a4043aad2384ce5ac62b6e