s3/printing: cppcheck avoid 'nullPointerArithmetic:' error

s3/printing: cppcheck avoid 'nullPointerArithmetic:' error

source3/printing/notify.c:94: error: nullPointerArithmetic: Pointer addition with NULL pointer. <--[cppcheck]
/home/samba/samba-pidl/source3/printing/notify.c:96: error: nullPointerArithmetic: Pointer addition with NULL pointer. <--[cppcheck]
/home/samba/samba-pidl/source3/printing/notify.c:103: error: nullPointerArithmetic: Pointer addition with NULL pointer. <--[cppcheck]

flatten_message function depends on behaviour of tdb_pack which will
return the bytes that would be written (without actually writing to the
buffer) if the bufsize passed is <=0. What we need to avoid here is the
default modification of buf (when it is initially NULL)

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andreas Schneider <asn@samba.org>