Use HB_SET_VALUE_INVALID consistently

diff --git a/src/hb-ot-layout-common-private.hh b/src/hb-ot-layout-common-private.hh
index cb66c81..c5e7f52 100644
--- a/src/hb-ot-layout-common-private.hh
+++ b/src/hb-ot-layout-common-private.hh

@@ -1057,7 +1057,7 @@
     if (klass == 0)
     {
       /* Match if there's any glyph that is not listed! */
-      hb_codepoint_t g = -1;
+      hb_codepoint_t g = HB_SET_VALUE_INVALID;
       if (!hb_set_next (glyphs, &g))
         return false;
       if (g < startGlyph)
@@ -1128,7 +1128,7 @@
     if (klass == 0)
     {
       /* Match if there's any glyph that is not listed! */
-      hb_codepoint_t g = (hb_codepoint_t) -1;
+      hb_codepoint_t g = HB_SET_VALUE_INVALID;
       for (unsigned int i = 0; i < count; i++)
       {
 	if (!hb_set_next (glyphs, &g))
@@ -1137,7 +1137,7 @@
 	  return true;
 	g = rangeRecord[i].end;
       }
-      if (g != (hb_codepoint_t) -1 && hb_set_next (glyphs, &g))
+      if (g != HB_SET_VALUE_INVALID && hb_set_next (glyphs, &g))
         return true;
       /* Fall through. */
     }

diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index fc5dd10..d9ba0f6 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc

@@ -957,7 +957,7 @@
   hb_set_t *copy = hb_set_create ();
   do {
     copy->set (glyphs);
-    for (hb_codepoint_t lookup_index = -1; hb_set_next (lookups, &lookup_index);)
+    for (hb_codepoint_t lookup_index = HB_SET_VALUE_INVALID; hb_set_next (lookups, &lookup_index);)
       hb_ot_layout_lookup_substitute_closure (font->face, lookup_index, glyphs);
   } while (!copy->is_equal (glyphs));
   hb_set_destroy (copy);