Slightly optimize UTF-8 parsing

src/hb-utf-private.hh

diff --git a/src/hb-utf-private.hh b/src/hb-utf-private.hh
index f89aa23..a77139f 100644
--- a/src/hb-utf-private.hh
+++ b/src/hb-utf-private.hh
@@ -44,8 +44,8 @@
 	     const uint8_t *end,
 	     hb_codepoint_t *unicode)
 {
-  uint8_t c = *text;
-  unsigned int mask, len;
+  hb_codepoint_t c = *text, mask;
+  unsigned int len;
 
   /* TODO check for overlong sequences? */