BN_clear() is used to destroy sensitive data such as keys when they
are no longer needed. It erases the memory used by B<a> and sets it
to the value 0.
+If B<a> is NULL, nothing is done.
BN_free() frees the components of the B<BIGNUM>, and if it was created
by BN_new(), also the structure itself. BN_clear_free() additionally