ldb: Fix segfault parsing new pack formats

ldb: Fix segfault parsing new pack formats

We need to check for the errors given by ldb_unpack() et al by preserving
the error code from kv_ctx->parser() called by tdb_parse_record() in
ltdb_parse_record().

Otherwise we will silently accept corrupt records and segfault later.

Likewise new pack formats will confuse the parser but not be
detected except by the incomplete struct ldb_message.

With this patch, the user will see a message like:

 Invalid data for index  DN=@BASEINFO

 Failed to connect to 'st/ad_dc/private/sam.ldb' with backend 'tdb': Unable to load ltdb cache records for backend 'ldb_tdb backend'
 Failed to connect to st/ad_dc/private/sam.ldb - Unable to load ltdb cache records for backend 'ldb_tdb backend'

This can be refined in the future by a specific check for
pack format versions in a higher caller, but this much is
needed regardless to detect corrupt records.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13959

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>