User tasks have empty nex_output().

This patch matches what is done in the development branch and
fixes the reported bug.

net_nex_output.cc

@@ -159,6 +159,15 @@ void NetSTask::nex_output(NexusSet&out)
 {
 }
 
+/*
+* Consider a task call to not have any outputs. This is not quite
+* right, we should be listing as outputs all the output ports, but for
+* the purposes that this method is used, this will do for now.
+*/
+void NetUTask::nex_output(NexusSet&out)
+{
+}
+
 void NetWhile::nex_output(NexusSet&out)
 {
       if (proc_ != 0)

netlist.h

@@ -2465,6 +2465,7 @@ class NetUTask  : public NetProc {
       const NetScope* task() const;
 
       virtual NexusSet* nex_input(bool rem_out = true);
+      virtual void nex_output(NexusSet&);
       virtual bool emit_proc(struct target_t*) const;
       virtual void dump(ostream&, unsigned ind) const;