Fix two memleaks in the encryption code

Fix two memleaks in the encryption code

ntlmssp_seal_packet creates its own signature data blob, which we then
have to free.

Jeremy, please check and merge appropriately (Yes, I'm asking you to do the
janitor work, I want you to *look* at this :-))

Volker
(cherry picked from commit 525af716cd52500a4e3dccf9eeb9ea85f86be7ca)