samdb: Fix build error with gcc8

samdb: Fix build error with gcc8

../source4/dsdb/samdb/ldb_modules/samldb.c: In function ‘samldb_add’:
../source4/dsdb/samdb/ldb_modules/samldb.c:424:6: error: ‘found’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   if (found) {
      ^
../source4/dsdb/samdb/ldb_modules/samldb.c:348:11: note: ‘found’ was declared here
  bool ok, found;
           ^~~~~

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13437

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>