Does not lock scratch space in RAM

[alexwebr]

The s20_crypt function uses automatic variables to store sensitive material (including, at various points, the key and the derived keystream blocks) during a cryptographic operation.

s20_crypt does not lock the memory ranges containing these variables in memory (using mlock or similar), and because the variables are automatic, does not make it easy for the user of this library to do so.

This could allow sensitive material (keys and keystream blocks) to be swapped out to disk.