ProveBatch() error when reorging

[kcalvinalvin]

I found a bug in ProveBatch() when I was trying to get multi-block proofs working.

When the forest has less than two leaves, you don't generate a BatchProof. This function is implemented with the following branch.

utreexo/accumulator/forestproofs.go

Lines 145 to 147 in e89de3f

	if f.data.size() < 2 {
	return bp, nil
	}

However, this doesn't take into consideration that f.data.size() returns the size of the allocated size of the forest, not the logical size. This is ok when you sync for the first time because f.data.size() will have the same value as the logical size. But when you resync after an undo, it returns an incorrect(bigger) value as you never reduce the allocated size during an undo.

forestproofs.go lines 145-147 should be modified to use the logical size, not the allocated size.