Set buffer content type to INVALID in hb_buffer_set_length(0)

Previously we were only setting this in hb_buffer_clear_contents(),
but set_length(0) is a valid way to reinitialize buffer to use with
new text.
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index bbf8ea0..d6c6fcb 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -1150,7 +1150,10 @@
   buffer->len = length;
 
   if (!length)
+  {
+    buffer->content_type = HB_BUFFER_CONTENT_TYPE_INVALID;
     buffer->clear_context (0);
+  }
   buffer->clear_context (1);
 
   return true;