[BUG] stick table purge failure if size less than 255

If table size is lower than 256, we can't force to purge old entries.
This patch should be backported to 1.4.

src/stick_table.c

@@ -158,7 +158,7 @@ struct stksess *stksess_new(struct stktable *t, struct stktable_key *key)
 		if ( t->nopurge )
 			return NULL;
 
-		if (!stktable_trash_oldest(t, t->size >> 8))
+		if (!stktable_trash_oldest(t, (t->size >> 8) + 1))
 			return NULL;
 	}