Fix typo!

Ouch!  Fortunately that function was unused.

src/hb-buffer-private.hh

@@ -77,7 +77,7 @@ struct hb_buffer_t {
   inline hb_glyph_position_t cur_pos (unsigned int i = 0) const { return pos[idx + i]; }
 
   inline hb_glyph_info_t &prev (void) { return out_info[out_len - 1]; }
-  inline hb_glyph_info_t prev (void) const { return info[out_len - 1]; }
+  inline hb_glyph_info_t prev (void) const { return out_info[out_len - 1]; }
 
   inline bool has_separate_output (void) const { return info != out_info; }