libwbclient: fix strict-overflow warning in wbcSidToString()

libwbclient: fix strict-overflow warning in wbcSidToString()

../../nsswitch/libwbclient/wbc_sid.c:83:5: error: assuming signed overflow does not occur when simplifying conditional [-Werror=strict-overflow]
  if (len+1 > sizeof(buf)) {
     ^

Even this would fail:
../../nsswitch/libwbclient/wbc_sid.c:83:5: error: assuming signed overflow does not occur when simplifying conditional [-Werror=strict-overflow]
  if (len >= sizeof(buf)) {
     ^

Note that this only seems to happen with gcc 7 and when -O3 and
-fvisibility=hidden are used together. E.g. in the opensuse151-samba-o3
builds.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14780

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>