If data is NULL, return the size needed to hold the derived key. No other API to do this, so document the behavior. Reviewed-by: Richard Levitte <levitte@openssl.org> (cherry picked from commit 5aed169305941fb1eba15fd4bacc0f998b0e43f7)
(cherry picked from commit 2b4ffc659eabec29f76821f0ac624a2b8c19e4c7)