[ldk-net] Correct pollfds array offset after reads

Previously, we were copying the pollfds array at an offset of one
into the handler thread's stack. However, when it was changed to
copying at a 0 offset (adding the pipe read fd at the end instead
of beginning), the disable-read handling code was not updated.

This leads to an assertion failure at runtime if LDK decides we
need to stop reading due to the outbound buffer of a peer being
full.

Author: TheBlueMatt

ldk-net/ldk_net.c

@@ -266,8 +266,8 @@ static void *sock_thread_fn(void* arg) {
 								if (*res.contents.result) {
 									lockres = pthread_mutex_lock(&handler->sockets_mutex);
 									assert(lockres == 0);
-									assert(handler->pollfds[i - 1].fd == pollfds[i].fd); // Only we change fd order!
-									handler->pollfds[i - 1].events = POLLOUT;
+									assert(handler->pollfds[i].fd == pollfds[i].fd); // Only we change fd order!
+									handler->pollfds[i].events = POLLOUT;
 									lockres = pthread_mutex_unlock(&handler->sockets_mutex);
 									assert(lockres == 0);
 								}