Set new code code part only if it is valid.

This fixes Sky Italia HD channels. When they send new
code word the one that is about to expire is set to 0
and this causes short decryption failures. Now if code
word is all zeroes it is not set.

camd.c

 		c->key->ts = c->key->ts_keyset.tv_sec;
 		ts->cw_last_warn = c->key->ts;
 
-		dvbcsa_key_set(c->key->cw    , c->key->csakey[0]);
-		dvbcsa_key_set(c->key->cw + 8, c->key->csakey[1]);
-
-		dvbcsa_bs_key_set(c->key->cw    , c->key->bs_csakey[0]);
-		dvbcsa_bs_key_set(c->key->cw + 8, c->key->bs_csakey[1]);
+		if (memcmp(c->key->cw, invalid_cw, 8) != 0) {
+			dvbcsa_key_set   (c->key->cw, c->key->csakey[0]);
+			dvbcsa_bs_key_set(c->key->cw, c->key->bs_csakey[0]);
+		}
+		if (memcmp(c->key->cw + 8, invalid_cw, 8) != 0) {
+			dvbcsa_key_set(c->key->cw + 8, c->key->csakey[1]);
+			dvbcsa_bs_key_set(c->key->cw + 8, c->key->bs_csakey[1]);
+		}
 	}
 
 	if (ts->ecm_cw_log) {