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.

module tangotests.loops1;

void main()
{
    size_t n;
    int x;
    ushort foo;
    for (n=0; n<8; n++,x++)
    {
        foo >>= 1;
    }
}