tcp: Fix assertion if tcp_kill_state() is called from tcp_alloc() to clean FIN_WAIT sockets

As reported on GitHub: https://github.com/espressif/esp-idf/issues/1194#issuecomment-585251932

src/core/tcp.c

@@ -1755,6 +1755,10 @@ tcp_kill_state(enum tcp_state state)
   struct tcp_pcb *pcb, *inactive;
   u32_t inactivity;
 
+#if !ESP_LWIP
+  LWIP_ASSERT("invalid state", (state == CLOSING) || (state == LAST_ACK));
+#endif
+
   inactivity = 0;
   inactive = NULL;
   /* Go through the list of active pcbs and get the oldest pcb that is in state