[cache] Fix cache coherency corner-case

If key_bits+value_bits-cache_bits==32 then -1 is ambiguous...

src/hb-cache.hh

@@ -36,7 +36,8 @@ template <unsigned int key_bits, unsigned int value_bits, unsigned int cache_bit
 struct hb_cache_t
 {
   static_assert ((key_bits >= cache_bits), "");
-  static_assert ((key_bits + value_bits - cache_bits <= 8 * sizeof (unsigned int)), "");
+  static_assert ((key_bits + value_bits - cache_bits <= 8 * sizeof (hb_atomic_int_t)), "");
+  static_assert (sizeof (hb_atomic_int_t) == sizeof (unsigned int));
 
   inline void init (void) { clear (); }
   inline void fini (void) {}
@@ -51,7 +52,8 @@ struct hb_cache_t
   {
     unsigned int k = key & ((1u<<cache_bits)-1);
     unsigned int v = values[k].get_relaxed ();
-    if ((v >> value_bits) != (key >> cache_bits))
+    if ((key_bits + value_bits - cache_bits == 8 * sizeof (hb_atomic_int_t) && v == (unsigned int) -1) ||
+	(v >> value_bits) != (key >> cache_bits))
       return false;
     *value = v & ((1u<<value_bits)-1);
     return true;