s3:lib: Fix undefined behavior in tdb_unpack()

Signed-off-by: Andreas Schneider <asn@samba.org>
24 jobs for asn/master-ubsan in 258 minutes and 10 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build_ctdb #126099238
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passed build_others #125902171
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passed build_samba #125902175
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passed build_samba_ad_dc #125902180
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passed build_samba_ad_dc_py3 #125902181
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passed build_samba_buildpy3_only #125902172
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passed build_samba_ctdb #125902170
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passed build_samba_fileserver #125902179
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passed build_samba_libs #125902166
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passed build_samba_libs_py3 #125902167
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passed build_samba_none_env #125902160
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passed build_samba_nopython #125902161
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passed build_samba_nt4 #125902177
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passed build_samba_nt4_py3 #125902178
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passed build_samba_o3 #125902164
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passed build_samba_purepy3-ad-dc-2 #125902174
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passed build_samba_purepy3-none_env #125902173
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passed build_samba_py3 #125902176
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passed build_samba_static #125902168
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passed build_samba_systemkrb5 #125902162
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passed build_samba_xc #125902163
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failed build_ctdb #126019021
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failed build_ctdb #125902169
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