inflation_layer: place .top() & .pop() calls together

Note that it doesn't make a real difference in this case (you are
inserting new stuff into the queue after the .top(), but it'll always
have a higher cost, so it's fine). Still, it's clearer like this, IMHO.
Merge pull request #171 from RainCT/hydro-devel

inflation_layer: place .top() & .pop() calls together