Sanity check DES_enc_write buffer length

Add a sanity check to DES_enc_write to ensure the buffer length provided
is not negative. Thanks to Kevin Wojtysiak (Int3 Solutions) and Paramjot
Oberoi (Int3 Solutions) for reporting this issue.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(cherry picked from commit 873fb39f)