Fix nested functions. My last patch was a little over-zealous in passing `undef`, it always passed `undef` to inner functions expecting a single context frame.

class A(T)
{
   void foo(void delegate (T) d) {}

   void bar()
   {
     foo(delegate void (T t) {});
   }
}

class B: A!(B) {}

class C: A!(C) {}