altcp_tcp: some fixes

- expose `altcp_tcp_setup()` so we can wrap altcp over existing tcp pcb.
- avoid calling tcp_close() with NULL pcb.
- free altcp_pcb struct when error callback called.

According to `mqtt_tcp_err_cb()` in src/apps/mqtt/mqtt.c, altcp socket should
work the same way than raw tcp socket. So freeing altcp_pcb ensure this.

src/include/lwip/altcp_tcp.h

@@ -53,6 +53,10 @@
 extern "C" {
 #endif
 
+/* setup a newly allocated altcp_pcb with existing tcp_pcb */
+struct tcp_pcb;
+void altcp_tcp_setup(struct altcp_pcb *conn, struct tcp_pcb *tpcb);
+
 struct altcp_pcb *altcp_tcp_new_ip_type(u8_t ip_type);
 
 #define altcp_tcp_new() altcp_tcp_new_ip_type(IPADDR_TYPE_V4)