A null sink BIO behaves in a similar manner to the Unix /dev/null
A null bio can be placed on the end of a chain to discard any data
passed through it.
A null sink is useful if, for example, an application wishes to digest some
data by writing through a digest bio but not send the digested data anywhere.
Since a BIO chain must normally include a source/sink BIO this can be achieved
by adding a null sink BIO to the end of the chain