Fixed #646 (crash on delete of library) (#685)

* Fixed a compiler warning

* Fixed #646 (crash on PCell Library "delete")

The issue was mainly caused by a "cleanup" call on
the library. Cleanup is supposed to remove excess
top level PCell variants. For libraries this is not
possible, as the library does not know which variants
are used and which are not.

In addition, some hardening against segfaults in
case of defect layouts has been applied.

src/laybasic/laybasic/layCellView.cc

@@ -59,6 +59,9 @@ LayoutHandle::LayoutHandle (db::Layout *layout, const std::string &filename)
     m_dirty (false),
     m_save_options_valid (false)
 {
+  //  layouts in the managed layouts space participate in spare proxy cleanup
+  layout->do_cleanup (true);
+
   file_watcher ().add_file (m_filename);
 
   if (! m_filename.empty ()) {