[set] Minor

In preparation to fix https://github.com/harfbuzz/harfbuzz/issues/863

Binary operations are sets are completely broken. Ouch.

src/hb-set-private.hh

@@ -430,19 +430,26 @@ struct hb_set_t
       {
 	a--;
 	b--;
-        Op::process (page_at (--count).v, page_at (a).v, other->page_at (b).v);
+	count--;
+	Op::process (page_at (count).v, page_at (a).v, other->page_at (b).v);
       }
       else if (page_map[a - 1].major > other->page_map[b - 1].major)
       {
 	a--;
 	if (Op::passthru_left)
-	  page_at (--count).v = page_at (a).v;
+	{
+	  count--;
+	  page_at (count).v = page_at (a).v;
+	}
       }
       else
       {
 	b--;
 	if (Op::passthru_right)
-	  page_at (--count).v = other->page_at (b).v;
+	{
+	  count--;
+	  page_at (count).v = other->page_at (b).v;
+	}
       }
     }
     if (Op::passthru_left)