BUG/MEDIUM: quic: fix null deref on error path in qc_conn_init()

When ctx is NULL, we go to the "err" label, which could dereference it.
No backport is needed.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 68d0f60..fea9a01 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -4164,7 +4164,7 @@
 	return 0;
 
  err:
-	if (ctx->wait_event.tasklet)
+	if (ctx && ctx->wait_event.tasklet)
 		tasklet_free(ctx->wait_event.tasklet);
 	pool_free(pool_head_quic_conn_ctx, ctx);
 	TRACE_DEVEL("leaving in error", QUIC_EV_CONN_NEW, conn);