s4:librpc/rpc: let dcerpc_ship_next_request() use a sig_size for a padded payload

s4:librpc/rpc: let dcerpc_ship_next_request() use a sig_size for a padded payload

The sig_size could differ depending on the aligment/padding.
So should use the same alignment as we use for the payload.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11061

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit fc249d542fcb8d043ae72eb7963d3a85eb79253a)