Fix reverse_range() for empty range

Fixes coretext notdef loop consisting of all default_ignorable glyphs

https://code.google.com/p/chromium/issues/detail?id=464755

src/hb-buffer.cc

diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 942177c..b9fe263 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -443,7 +443,7 @@
 {
   unsigned int i, j;
 
-  if (start == end - 1)
+  if (end - start < 2)
     return;
 
   for (i = start, j = end - 1; i < j; i++, j--) {