dfs_server: Fix debug statement if searched_site is NULL

dfs_server: Fix debug statement if searched_site is NULL

In file included from source4/include/includes.h:61,
                 from dfs_server/dfs_server_ad.c:21:
dfs_server/dfs_server_ad.c: In function ‘get_dcs.constprop’:
lib/util/debug.h:200:12: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  200 |        && (dbgtext body) )
      |           ~^~~~~~~~~~~~~
dfs_server/dfs_server_ad.c:462:25: note: in expansion of macro ‘DEBUG’
  462 |                         DEBUG(2,(__location__ ": Site: %s %s\n",
      |                         ^~~~~

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