Add FLAG_UNSAFE()

Unused right now.

src/hb-private.hh

diff --git a/src/hb-private.hh b/src/hb-private.hh
index dc67b3a..8112934 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -850,6 +850,7 @@
  */
 #define FLAG(x) (ASSERT_STATIC_EXPR_ZERO ((x) < 32) + (1U << (x)))
 #define FLAG_SAFE(x) (1U << (x))
+#define FLAG_UNSAFE(x) ((x) < 32 ? FLAG_SAFE(x) : 0)
 #define FLAG_RANGE(x,y) (ASSERT_STATIC_EXPR_ZERO ((x) < (y)) + FLAG(y+1) - FLAG(x))