lib:krb5_wrap: Fix debug statements when princ_s is NULL

lib:krb5_wrap: Fix debug statements when princ_s is NULL

In file included from source4/include/includes.h:61,
                 from lib/krb5_wrap/krb5_samba.c:23:
lib/krb5_wrap/krb5_samba.c: In function ‘smb_krb5_kt_seek_and_delete_old_entries’:
lib/util/debug.h:200:12: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  200 |        && (dbgtext body) )
      |           ~^~~~~~~~~~~~~
lib/krb5_wrap/krb5_samba.c:1753:25: note: in expansion of macro ‘DEBUG’
 1753 |                         DEBUG(5, (__location__ ": Saving previous (kvno %d) "
      |                         ^~~~~
lib/util/debug.h:200:12: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  200 |        && (dbgtext body) )
      |           ~^~~~~~~~~~~~~
lib/krb5_wrap/krb5_samba.c:1763:25: note: in expansion of macro ‘DEBUG’
 1763 |                         DEBUG(5, (__location__ ": Saving entry with kvno [%d] "
      |                         ^~~~~
lib/util/debug.h:200:12: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  200 |        && (dbgtext body) )
      |           ~^~~~~~~~~~~~~
lib/krb5_wrap/krb5_samba.c:1769:17: note: in expansion of macro ‘DEBUG’
 1769 |                 DEBUG(5, (__location__ ": Found old entry for principal: %s "
      |                 ^~~~~
lib/util/debug.h:200:12: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  200 |        && (dbgtext body) )
      |           ~^~~~~~~~~~~~~
lib/krb5_wrap/krb5_samba.c:1787:17: note: in expansion of macro ‘DEBUG’
 1787 |                 DEBUG(5, (__location__ ": removed old entry for principal: "
      |                 ^~~~~

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