s3 rpc_client: Fix Asan stack use after scope

Fixes Asan error:

==1924==ERROR: AddressSanitizer: stack-use-after-scope on address
    0x7ffe63f873d0 at pc 0x7fb99dae1733 bp 0x7ffe63f86a00 sp 0x7ffe63f861a8
READ of size 24 at 0x7ffe63f873d0 thread T0
    #0 0x7fb99dae1732  (/usr/lib/x86_64-linux-gnu/libasan.so.4+0x79732)
    #1 0x7fb99cfe5549 in memcpy
        /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
    #2 0x7fb99cfe5549 in ndr_push_bytes
        ../../librpc/ndr/ndr_basic.c:729
    #3 0x7fb99cfe5646 in ndr_push_array_uint8
        ../../librpc/ndr/ndr_basic.c:754
    #4 0x7fb99a69dd1b in ndr_push_netr_ChallengeResponse
        librpc/gen_ndr/ndr_netlogon.c:462
    #5 0x7fb99a6c5fab in ndr_push_netr_NetworkInfo
        librpc/gen_ndr/ndr_netlogon.c:556
    #6 0x7fb99a6c749d in ndr_push_netr_LogonLevel
         librpc/gen_ndr/ndr_netlogon.c:783
    #7 0x7fb99a7222de in ndr_push_netr_LogonSamLogonEx
         librpc/gen_ndr/ndr_netlogon.c:16547
    #8 0x7fb99c982c97 in dcerpc_binding_handle_call_send
         ../../librpc/rpc/binding_handle.c:416

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13936

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Fri May 10 10:02:21 UTC 2019 on sn-devel-184

(cherry picked from commit a5d1df4a)
22 jobs for gary-bug-13936-v4-10 in 206 minutes and 49 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build_ctdb #212550267
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passed build_others #212550269
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passed build_samba_fileserver #212550273
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passed build_samba_nopython #212550251
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passed build_samba_o3 #212550255
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passed build_samba_static #212550266
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passed build_samba_systemkrb5 #212550252
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passed build_samba_xc #212550254
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