Fixed crash in constant folding optimization

theano/tensor/basic.py

@@ -3370,6 +3370,10 @@ class Alloc(gof.Op):
         return self.make_node(eval_points[0], *inputs[1:]).outputs
 
     def do_constant_folding(self, node):
+        if not hasattr(node.outputs[0], 'clients'):
+            # If there are no clients then there is no point doing constant
+            # folding.
+            return False
         for client in node.outputs[0].clients:
             if client[0] == 'output':
                 # If the output is a constant, it will have to be deepcopied