Bug in Hangul composition

[behdad]

Patch and test:

diff --git a/src/hb-ucdn/ucdn.c b/src/hb-ucdn/ucdn.c
index 30747fea..f7b33d64 100644
--- a/src/hb-ucdn/ucdn.c
+++ b/src/hb-ucdn/ucdn.c
@@ -163,7 +163,8 @@ static int hangul_pair_decompose(uint32_t code, uint32_t *a, uint32_t *b)
 
 static int hangul_pair_compose(uint32_t *code, uint32_t a, uint32_t b)
 {
-    if (a >= SBASE && a < (SBASE + SCOUNT) && b >= TBASE && b < (TBASE + TCOUNT)) {
+    if (a >= SBASE && a < (SBASE + SCOUNT) && b > TBASE && b < (TBASE + TCOUNT) &&
+       !((a - SBASE) % TCOUNT)) {
         /* LV,T */
         *code = a + (b - TBASE);
         return 3;
diff --git a/test/api/test-unicode.c b/test/api/test-unicode.c
index 6195bb28..0587c6e7 100644
--- a/test/api/test-unicode.c
+++ b/test/api/test-unicode.c
@@ -755,6 +755,10 @@ test_unicode_normalization (gconstpointer user_data)
   g_assert (hb_unicode_compose (uf, 0xCE20, 0x11B8, &ab) && ab == 0xCE31);
   g_assert (hb_unicode_compose (uf, 0x110E, 0x1173, &ab) && ab == 0xCE20);
 
+  g_assert (!hb_unicode_compose (uf, 0xAC00, 0x11A7, &ab));
+  g_assert (hb_unicode_compose (uf, 0xAC00, 0x11A8, &ab) && ab == 0xAC01);
+  g_assert (!hb_unicode_compose (uf, 0xAC01, 0x11A8, &ab));
+
 
   /* Test decompose() */