[bit-set-invertible] Fix previous() iteration
diff --git a/src/hb-bit-set-invertible.hh b/src/hb-bit-set-invertible.hh
index 3b1d9df..5be6e45 100644
--- a/src/hb-bit-set-invertible.hh
+++ b/src/hb-bit-set-invertible.hh
@@ -239,7 +239,7 @@
     auto old = *codepoint;
     auto v = old;
     s.previous (&v);
-    if (old - 1 > v)
+    if (old - 1 > v || v == INVALID)
     {
       *codepoint = old - 1;
       return true;