s4:torture: make rpc.handles.random-assoc test more robust

s4:torture: make rpc.handles.random-assoc test more robust

We've seen failures like this:

  RPC-HANDLE-RANDOM-ASSOC
  connect samr pipe1
  pipe1 uses assoc_group_id[0x00000001]
  connect samr pipe2 with assoc_group_id[0xFFFFFFFF]- should fail
  Failed to bind to uuid 12345778-1234-abcd-ef00-0123456789ac for
    ncacn_np:localdc[\pipe\samr,bigendian,assoc_group_id=0xffffffff,
    abstract_syntax=12345778-1234-abcd-ef00-0123456789ac/0x00000001] NT_STATUS_UNSUCCESSFUL
  connect samr pipe3 with assoc_group_id[0x00000000]- should fail
  UNEXPECTED(failure): samba4.rpc.handles on ncacn_np with bigendian.random-assoc(ad_dc_default)
  REASON: Exception: Exception: ../../source4/torture/rpc/handles.c:546:
    status was NT_STATUS_OK, expected NT_STATUS_UNSUCCESSFUL: opening samr pipe3

Prevent that it wraps to 0x00000000

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>