(run_pre_post_conversion_on_str): Use delete_all_overlays.

src/coding.c

@@ -5998,12 +5998,13 @@ run_pre_post_conversion_on_str (str, coding, encodep)
   buffer = Fget_buffer_create (build_string (" *code-converting-work*"));
   buf = XBUFFER (buffer);
 
+  delete_all_overlays (buf);
   buf->directory = current_buffer->directory;
   buf->read_only = Qnil;
   buf->filename = Qnil;
   buf->undo_list = Qt;
-  buf->overlays_before = NULL;
-  buf->overlays_after = NULL;
+  eassert (buf->overlays_before == NULL);
+  eassert (buf->overlays_after == NULL);
 
   set_buffer_internal (buf);
   /* We must insert the contents of STR as is without