cmOutputConverter: Assert construction with a valid snapshot

We unconditionally use information from the snapshot so it must be
valid.

Source/cmOutputConverter.cxx

@@ -22,6 +22,7 @@
 cmOutputConverter::cmOutputConverter(cmState::Snapshot snapshot)
   : StateSnapshot(snapshot), LinkScriptShell(false)
 {
+  assert(this->StateSnapshot.IsValid());
 }
 
 //----------------------------------------------------------------------------