Tile Expanded Integrity Check, Still Lightweight

At the moment we have a pretty relaxed check when we load a graphtile into memory and initialize its internal pointers. We do 2 things:

1. we check that the tile is large enough to contain at least a tile header
2. we check that the size of the tile matches the size reported in the tile header

We should do just a little bit more, specificaly, when we do pointer arithmetic to set the various pointers to the various data structures in the tile we should also make sure that none of them go past the end of the the address range that the tile is supposed to encompass (based on its size)