Require topics>0 in mosquitto_[un]subscribe*().

ChangeLog.txt

@@ -134,6 +134,10 @@ Client library:
   `%u` patterns for client id / username substitution.
 - Performance: reduce memory allocations when sending packets.
 - Reintroduce threading support for Windows. Closes #1509.
+- `mosquitto_subscribe*()` now returns MOSQ_ERR_INVAL if an empty string is
+  passed as a topic filter.
+- `mosquitto_unsubscribe*()` now returns MOSQ_ERR_INVAL if an empty string is
+  passed as a topic filter.
 
 Clients:
 - Add `-W` timeout support to Windows.

lib/actions_subscribe.c

@@ -69,6 +69,9 @@ int mosquitto_subscribe_multiple(struct mosquitto *mosq, int *mid, int sub_count
 	for(i=0; i<sub_count; i++){
 		if(mosquitto_sub_topic_check(sub[i])) return MOSQ_ERR_INVAL;
 		slen = (int)strlen(sub[i]);
+		if(slen == 0){
+			return MOSQ_ERR_INVAL;
+		}
 		if(mosquitto_validate_utf8(sub[i], slen)) return MOSQ_ERR_MALFORMED_UTF8;
 		remaining_length += 2+(uint32_t)slen + 1;
 	}

lib/actions_unsubscribe.c

@@ -67,6 +67,7 @@ int mosquitto_unsubscribe_multiple(struct mosquitto *mosq, int *mid, int sub_cou
 	for(i=0; i<sub_count; i++){
 		if(mosquitto_sub_topic_check(sub[i])) return MOSQ_ERR_INVAL;
 		slen = (int)strlen(sub[i]);
+		if(slen == 0) return MOSQ_ERR_INVAL;
 		if(mosquitto_validate_utf8(sub[i], slen)) return MOSQ_ERR_MALFORMED_UTF8;
 		remaining_length += 2U + (uint32_t)slen;
 	}