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.

src/hb-buffer-private.hh

@@ -181,7 +181,7 @@ struct hb_buffer_t {
   HB_INTERNAL bool enlarge (unsigned int size);
 
   inline bool ensure (unsigned int size)
-  { return likely (size < allocated) ? true : enlarge (size); }
+  { return likely (!size || size < allocated) ? true : enlarge (size); }
 
   HB_INTERNAL bool make_room_for (unsigned int num_in, unsigned int num_out);
   HB_INTERNAL bool shift_forward (unsigned int count);

src/hb-buffer.cc

@@ -1150,7 +1150,10 @@ hb_buffer_set_length (hb_buffer_t  *buffer,
   buffer->len = length;
 
   if (!length)
+  {
+    buffer->content_type = HB_BUFFER_CONTENT_TYPE_INVALID;
     buffer->clear_context (0);
+  }
   buffer->clear_context (1);
 
   return true;