dcerpc: correctly initialize certain struct fields

Bug: 10482
Change-Id: I4d39d6a79d4ab89685a17a0c622c6181e4fada3d
Reviewed-on: https://code.wireshark.org/review/4233
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit b8065a5b7d653aca53033900c27e64d835d57ffd)
Reviewed-on: https://code.wireshark.org/review/4235