Eliminate failure mode where streamDefTimestep is called before streamDefVlist.

src/stream.c

@@ -1293,6 +1293,11 @@ cdiStreamDefTimestep_(stream_t *streamptr, int tsID)
   if (CDI_Debug) Message("streamID = %d  tsID = %d", streamptr->self, tsID);
 
   const int vlistID = streamptr->vlistID;
+  if (vlistID == CDI_UNDEFID)
+    Error("Must not call streamDefTimestep for stream (ID=%d)"
+          " with (not yet) defined vlist",
+          streamptr->self);
+
   const int timeIsVarying = vlistHasTime(vlistID);
   const int taxisID = vlistInqTaxis(vlistID);