bound.next_edge: Not initialized before first call

[DiabloRusso]

bound.hpp: Add next_edge initialization
snap_rounding.hpp: In insert_local_minima_into_ABL_hot_pixel with
current_edge also initialized next_edge

[springmeyer]

@DiabloRusso thank you for finding this. This helped me realized that I'd be wrongly assuming for quite some time now that the -Weffc++ flag in clang++ would catch this problem. I've run that locally with wagyu and clang++ (via -Weverything) but did not see any uninitialized members. Turns out that -Weffc++ catches this problem with g++ but not clang++ because clang++'s warning just maps to -Wnon-virtual-dtor (https://clang.llvm.org/docs/DiagnosticsReference.html#weffc).

But I pushed a test branch enabling it on travis, where we do test with g++, and sure enough this bug is caught (since we turn warnings into errors): https://travis-ci.org/mapbox/wagyu/jobs/202717641#L348

[springmeyer]

Rebased against master, the -Weffc++ builds now pass: https://travis-ci.org/mapbox/wagyu/builds/202725387. So it looks like @DiabloRusso uncovered the only spot this was happening.