Fix UTF-8 backward iteration

Ouch!
diff --git a/src/hb-utf-private.hh b/src/hb-utf-private.hh
index 8cde827..ae36ef8 100644
--- a/src/hb-utf-private.hh
+++ b/src/hb-utf-private.hh
@@ -77,8 +77,8 @@
 	     const uint8_t *start,
 	     hb_codepoint_t *unicode)
 {
-  const uint8_t *end = text;
-  while (start < text && (*--text & 0xc0) == 0x80 && end - text < 4)
+  const uint8_t *end = text--;
+  while (start < text && (*text & 0xc0) == 0x80 && end - text < 4)
     text--;
 
   hb_codepoint_t c = *text, mask;