Correct the probe heap comparison function

Fix the probe scheduler heap comparison function to be consistent with
regard to different running state of the two arguments. With this fix,
probes that are not running will always bubble to the top before those
that are already running.

bin/varnishd/cache/cache_backend_probe.c

@@ -696,8 +696,8 @@ vbp_cmp(void *priv, const void *a, const void *b)
 	CAST_OBJ_NOTNULL(aa, a, VBP_TARGET_MAGIC);
 	CAST_OBJ_NOTNULL(bb, b, VBP_TARGET_MAGIC);
 
-	if (aa->running && !bb->running)
-		return (0);
+	if ((aa->running == 0) != (bb->running == 0))
+		return (aa->running == 0);
 
 	return (aa->due < bb->due);
 }