Reset websocket subprotocol for every request

See #1091

CREDITS.md

@@ -162,6 +162,7 @@
 * Ponnuvel Palaniyappan
 * qinch
 * qinchao
+* r-j-s
 * Radoslaw Zarzynski
 * Red54
 * Retallack Mark mark.retallack

src/civetweb.c

@@ -19281,6 +19281,7 @@ init_connection(struct mg_connection *conn)
 	conn->data_len = 0;
 	conn->handled_requests = 0;
 	conn->connection_type = CONNECTION_TYPE_INVALID;
+	conn->request_info.acceptedWebSocketSubprotocol = NULL;
 	mg_set_user_connection_data(conn, NULL);
 
 #if defined(USE_SERVER_STATS)