Fix casts to template types not checking for non-values

This would previously provoke an ICE

Author: lwaern-intel

py/dml/ctree.py

@@ -3733,6 +3733,8 @@ def read(self):
                 + '(_traitref_t) {_object_vtables[__id.id], __id};})')
 
 def mkTraitUpcast(site, sub, parent):
+    if isinstance(sub, NonValue):
+        raise sub.exc()
     typ = safe_realtype(sub.ctype())
     assert dml.globals.object_trait
     if isinstance(typ, TTrait):

test/1.4/errors/T_ENVAL.dml

@@ -46,6 +46,9 @@ method init() {
     b.r1;
     /// ERROR ENVAL
     b.r2.f;
+
+    /// ERROR ENVAL
+    cast([], t);
 }
 
 // dev is not a value in 1.4.  Need to capture this with a rather unnatural