Don't crash looking for delay_type of empty task.

It is legal for a task to have no definition. In that case, the
delay_type calculations (used to detect infinite loops) can assume
that an empty definition is a no-op and return NO_DELAY.

netlist.cc

@@ -2649,6 +2649,7 @@ DelayType NetRepeat::delay_type() const
 
 DelayType NetTaskDef::delay_type() const
 {
+      if (proc_ == 0) return NO_DELAY;
       return proc_->delay_type();
 }