librpc:ndr: Fix undefined behavior in ndr_basic

librpc:ndr: Fix undefined behavior in ndr_basic

librpc/ndr/ndr_basic.c:723:2: runtime error: null pointer passed as
argument 2, which is declared to never be null

The following triggered the undefined behavior:

(gdb) bt
    at librpc/gen_ndr/ndr_drsuapi.c:2318
    fn=0x7ffff6e72983 <ndr_push_drsuapi_DsReplicaObjectIdentifier3Binary>) at ../../librpc/ndr/ndr.c:1337
    at ../../source4/dsdb/schema/schema_syntax.c:2136
    drs_str=<optimized out>) at ../../source4/dsdb/schema/tests/schema_syntax.c:122
    already_setup=<optimized out>, restricted=restricted@entry=0x0) at ../../lib/torture/torture.c:442
    at ../../lib/torture/torture.c:507
    suite=0x5555563d9490, matched=0x7fffffffcef7) at ../../source4/torture/smbtorture.c:93
    matched=0x7fffffffcef7) at ../../source4/torture/smbtorture.c:95
    at ../../source4/torture/smbtorture.c:143
(gdb) f 1
1335            NDR_CHECK(ndr_push_bytes(ndr, blob.data, blob.length));
(gdb) p blob
$2 = {data = 0x0, length = 0}

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Gary Lockyer <gary@samba.org>
Autobuild-Date(master): Thu Dec  6 08:48:28 CET 2018 on sn-devel-144