MINOR: mux-quic: fix a possible null dereference in qc_timeout_task

The qcc instance should be tested as it is implied by a previous test
that it may be NULL. In this case, qc_timeout_task can be stopped.

This should fix github issue #1559.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 2ed269e..c01fcbf 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -427,6 +427,10 @@
 
 	fprintf(stderr, "%s: timeout\n", __func__);
 	task_destroy(t);
+
+	if (!qcc)
+		return NULL;
+
 	qcc->task = NULL;
 
 	if (qcc_is_dead(qcc))