poll: support 0 nfds

kernel-src/sys/syscalls/poll.c

@@ -31,7 +31,9 @@ syscallret_t syscall_poll(context_t *, pollfd_t *fds, size_t nfds, int timeoutms
 	memcpy(fdsbuff, fds, fdsbuffsize);
 
 	polldesc_t desc = {0};
-	ret.errno = poll_initdesc(&desc, nfds);
+	// when nfds is 0, poll is used as a sleep.
+	// therefore, initialize the desc size to one descriptor in that case.
+	ret.errno = poll_initdesc(&desc, nfds == 0 ? 1 : nfds);
 	if (ret.errno) {
 		goto cleanup;
 	}