Prevent segfault if incorrect blob column is passed to openBlob()

[mreis1]

Example:

// a)
const blob = results[0][0] as Blob; // This is the correct way to pass in the blob reference
// b)
const blob = results[0] as Blob; // But, an incorrect value such as this one, will cause a segfault
const blobStream = await attachment.openBlob(transaction, blob);