[sfnt] Return correct number of named instances for TTCs.

Without this patch, requesting information for face index N returned
the data for face index N+1 (or index 0).

* src/sfnt/sfobjs.c (sfnt_init_face): Correctly adjust `face_index'
for negative `face_instance_index' values.

ChangeLog

@@ -1,3 +1,13 @@
+2017-01-01  Werner Lemberg  <wl@gnu.org>
+
+	[sfnt] Return correct number of named instances for TTCs.
+
+	Without this patch, requesting information for face index N returned
+	the data for face index N+1 (or index 0).
+
+	* src/sfnt/sfobjs.c (sfnt_init_face): Correctly adjust `face_index'
+	for negative `face_instance_index' values.
+
 2016-12-31  Werner Lemberg  <wl@gnu.org>
 
 	*/*: Use hex numbers for errors in tracing messages.

src/sfnt/sfobjs.c

@@ -909,6 +909,10 @@
 
     face_index = FT_ABS( face_instance_index ) & 0xFFFF;
 
+    /* value -(N+1) requests information on index N */
+    if ( face_instance_index < 0 )
+      face_index--;
+
     if ( face_index >= face->ttc_header.count )
     {
       if ( face_instance_index >= 0 )