secur32: Properly handle NULL pInput argument for DTLS contexts in schan_InitializeSecurityContextW.

When a NULL pInput argument is passed into InitializeSecurityContextW
for an existing DTLS context, we need to retransmit the last handshake
packet.
Signed-off-by: Connor McAdams <cmcadams@codeweavers.com>
Signed-off-by: Hans Leidekker <hans@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>