[HB] Fix buffer enlargement.  Ouch

src/hb-buffer.c

diff --git a/src/hb-buffer.c b/src/hb-buffer.c
index 31b6c0f..d2ec33d 100644
--- a/src/hb-buffer.c
+++ b/src/hb-buffer.c
@@ -244,7 +244,7 @@
   unsigned int properties;
   unsigned int cluster;
 
-  if (buffer->out_string == buffer->in_string &&
+  if (buffer->out_string != buffer->in_string ||
       buffer->out_pos + num_out > buffer->in_pos + num_in)
   {
     hb_buffer_ensure_separate (buffer, buffer->out_pos + num_out);