[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.
diff --git a/src/stick_table.c b/src/stick_table.c
index 78a0df7..cb9b6b3 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -158,7 +158,7 @@
 		if ( t->nopurge )
 			return NULL;
 
-		if (!stktable_trash_oldest(t, t->size >> 8))
+		if (!stktable_trash_oldest(t, (t->size >> 8) + 1))
 			return NULL;
 	}