Check that TCP connections aren't dropped in callbacks.

They aren't, but it's unclear whether it's structurally impossible.

toxcore/TCP_connection.c

@@ -29,6 +29,8 @@
 
 #include "util.h"
 
+#include <assert.h>
+
 
 struct TCP_Connections {
     DHT *dht;
@@ -1397,6 +1399,9 @@ static void do_tcp_conns(TCP_Connections *tcp_c, void *userdata)
                 /* callbacks can change TCP connection address. */
                 tcp_con = get_tcp_connection(tcp_c, i);
 
+                // Make sure the TCP connection wasn't dropped in any of the callbacks.
+                assert(tcp_con != NULL);
+
                 if (tcp_con->connection->status == TCP_CLIENT_DISCONNECTED) {
                     if (tcp_con->status == TCP_CONN_CONNECTED) {
                         reconnect_tcp_relay_connection(tcp_c, i);