tdb: reset tdb->fd to -1 in tdb_close()
    So that erroneous double tdb_close() calls do not try to close() same
    fd again. This is like SAFE_FREE() but for fd.
    Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    (cherry picked from samba commit 
b4424f8234a78a79fb2d71d46ca208b4f12e0f9e)
    Signed-off-by: Stefan Metzmacher <metze@samba.org>
                        tdb_munmap(tdb);
        }
        SAFE_FREE(tdb->name);
-       if (tdb->fd != -1)
+       if (tdb->fd != -1) {
                ret = close(tdb->fd);
+               tdb->fd = -1;
+       }
        SAFE_FREE(tdb->lockrecs);
 
        /* Remove from contexts list */