[map] Add hb_map_filter_invalid

Use it in one place.

src/OT/Layout/Common/Coverage.hh

@@ -147,8 +147,8 @@ struct Coverage
     TRACE_SUBSET (this);
     auto it =
     + iter ()
-    | hb_filter (c->plan->glyph_map_gsub)
     | hb_map_retains_sorting (c->plan->glyph_map_gsub)
+    | hb_filter (hb_map_filter_invalid)
     ;
 
     // Cache the iterator result as it will be iterated multiple times

src/hb-map.hh

@@ -451,6 +451,8 @@ struct hb_map_t : hb_hashmap_t<hb_codepoint_t,
   hb_map_t (const Iterable &o) : hashmap (o) {}
 };
 
+static inline bool hb_map_filter_invalid (hb_codepoint_t _) { return _ != HB_MAP_VALUE_INVALID; }
+
 template <typename K, typename V>
 static inline
 hb_hashmap_t<K, V>* hb_hashmap_create ()