Add API test for hb_ot_color_get_color_layers

src/dump-emoji.cc

diff --git a/src/dump-emoji.cc b/src/dump-emoji.cc
index e6fddee..14376a8 100644
--- a/src/dump-emoji.cc
+++ b/src/dump-emoji.cc
@@ -91,7 +91,8 @@
 {
   unsigned int upem = hb_face_get_upem (face);
 
-  for (hb_codepoint_t gid = 0; gid < hb_face_get_glyph_count (face); ++gid)
+  unsigned glyph_count = hb_face_get_glyph_count (face);
+  for (hb_codepoint_t gid = 0; gid < glyph_count; ++gid)
   {
     unsigned int num_layers = hb_ot_color_get_color_layers (face, gid, 0, nullptr, nullptr, nullptr);
     if (!num_layers)