Fix use-after-free in overlay_clear

src/overlay.c

@@ -23,9 +23,11 @@ void overlay_done(Overlay* overlay) {
 
 int overlay_clear(Overlay* overlay) {
     int n_cleared = 0;
+    overlay_message* next = NULL;
     for (   overlay_message* message = overlay->messages;
             NULL != message;
-            message = message->next) {
+            message = next) {
+        next = message->next;
         free(message->string);
         free(message);
         ++n_cleared;