s3/mdssvc: missing assignment in sl_pack_float

s3/mdssvc: missing assignment in sl_pack_float

Spotted by -Werror=maybe-uninitialized:

../source3/rpc_server/mdssvc/marshalling.c: In function ‘sl_pack_float’:
../source3/rpc_server/mdssvc/marshalling.c:171:11: error:
‘ieee_fp_union.w’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
  offset = sl_push_uint64_val(buf, offset, bufsize, ieee_fp_union.w);

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12991

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>