BIO_s_null() returns the null sink BIO method. Data written to the
null sink is discarded, reads return EOF.
NOTES
A null sink BIO behaves in a similar manner to the Unix /dev/null
device.
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
