m has been initialized with 4 (m = 4)
Dim p(m) As Point means that you allocated 5 memory slots big enough to hold a point structure.

But each of it is still has the value of Nothing. You should assign a new Point to each of these elements before referring to them.
It's like you have 5 boxes for 5 pairs of shoes. Having the boxes doesn't always mean that you actually have the shoes. In fact, all your boxes are empty. Before you could use each box, you should place a new pair of shoes into each one.