Windows: Fix possible crash when client disconnects.

Closes #1137. Thanks to Kris Mattheus.

ChangeLog.txt

@@ -33,6 +33,7 @@ Broker:
 - Allow broker to always restart on Windows when using `log_dest file`. Closes
   #1080.
 - Fix Will not being sent for Websockets clients. Closes #1143.
+- Windows: Fix possible crash when client disconnects. Closes #1137.
 
 Library:
 - Fix TLS connections not working over SOCKS.

src/websockets.c

@@ -247,7 +247,7 @@ static int callback_mqtt(struct libwebsocket_context *context,
 			}
 			mosq = u->mosq;
 			if(mosq){
-				if(mosq->sock > 0){
+				if(mosq->sock != INVALID_SOCKET){
 					HASH_DELETE(hh_sock, db->contexts_by_sock, mosq);
 					mosq->sock = INVALID_SOCKET;
 					mosq->pollfd_index = -1;