sml: Delay freeing of trimmed segments always

sml_trimstore() already delays freeing of a replaced last, smaller segment for
the reallocation case at the bottom of the function: Because a concurrently
running iterator might have already taken a reference on a to-be-replaced
segment, it can not be freed immediately, but rather is kept around until the
busy object is no more.

This trivial change applies the same also for a segment which turns out to be
unneeded because writing the object ended with a zero length in this segment.

Simple, but consequential, see next commit

bin/varnishd/storage/storage_simple.c

@@ -560,7 +560,8 @@ sml_trimstore(struct worker *wrk, struct objcore *oc)
 		Lck_Lock(&oc->boc->mtx);
 		VTAILQ_REMOVE(&o->list, st, list);
 		Lck_Unlock(&oc->boc->mtx);
-		sml_stv_free(stv, st);
+		/* sml_bocdone frees this */
+		oc->boc->stevedore_priv = st;
 		return;
 	}