Fix opcode result tuple validation

The tuple validation for opcode results failed to assert if a result
was a tuple before attempting to check its length and types. This
patch adds the missing "Is it a tuple?" check.
Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

lib/ht.py

@@ -433,7 +433,7 @@ def TTupleOf(*val_types):
 
   """
   desc = WithDesc("Tuple of %s" % (Parens(val_types), ))
-  return desc(TAnd(TIsLength(len(val_types)), TItems(val_types)))
+  return desc(TAnd(TTuple, TIsLength(len(val_types)), TItems(val_types)))
 
 
 def TSetOf(val_type):