[buffer] Rewind cursor in clear_output()
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 7aa37ea..7ca988d 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -297,6 +297,7 @@
   have_output = true;
   have_positions = false;
 
+  idx = 0;
   out_len = 0;
   out_info = info;
 }