fix missing initialization

... introduced with 3bb8b84c:

in Pool_Work_Thread(), we could break out of the for (i = 0; i <
TASK_QUEUE__END; i++) loop with tp set to the value from the previous
iteration of the top while() loop where if should have been NULL (for no
task found).

Noticed staring at #3192 - unclear yet if related

bin/varnishd/cache/cache_wrk.c

@@ -335,7 +335,7 @@ pool_kiss_of_death(struct worker *wrk, void *priv)
 static void
 Pool_Work_Thread(struct pool *pp, struct worker *wrk)
 {
-	struct pool_task *tp = NULL;
+	struct pool_task *tp;
 	struct pool_task tpx, tps;
 	vtim_real tmo;
 	int i, reserve;
@@ -344,6 +344,7 @@ Pool_Work_Thread(struct pool *pp, struct worker *wrk)
 	wrk->pool = pp;
 	while (1) {
 		CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
+		tp = NULL;
 
 		WS_Reset(wrk->aws, 0);
 		AZ(wrk->vsl);