s3:mdssvc: avoid strncpy when marshalling strings

s3:mdssvc: avoid strncpy when marshalling strings

Avoids failure when at O3 level:

  [2082/4232] Compiling source3/rpc_server/mdssvc/marshalling.c

  ==> /builds/samba-team/devel/samba/samba-o3.stderr <==
  In file included from /usr/include/string.h:494,
                   from /usr/include/bsd/string.h:30,
                   from ../../lib/tevent/../replace/replace.h:164,
                   from ../../source3/include/includes.h:23,
                   from ../../source3/rpc_server/mdssvc/marshalling.c:21:
  In function ‘strncpy’,
      inlined from ‘sl_pack_string’ at ../../source3/rpc_server/mdssvc/marshalling.c:493:2,
      inlined from ‘sl_pack_loop’ at ../../source3/rpc_server/mdssvc/marshalling.c:607:13:
  /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ output
  truncated before terminating nul copying as many bytes from a string as its
  length [-Werror=stringop-truncation]
    106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../../source3/rpc_server/mdssvc/marshalling.c: In function ‘sl_pack_loop’:
  ../../source3/rpc_server/mdssvc/marshalling.c:458:8: note: length computed here
    458 |  len = strlen(s);
        |        ^~~~~~~~~
  cc1: all warnings being treated as errors

Marshalled strings are not 0 terminated.

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