In #37 we've decided to remove length check, because smart contract would
fail on casting `nil` value from storage to `[]byte` producing FAULT state.
Apparently it does not fail, so we have to check length explicitly.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
Container get wrapper implements container.Source interface
so it can be used in object service as container storage.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
