Fix the error check in FloodDecodePositions

d'oh

This is functionally identical in all *happy path* cases (index 0 was
being falsely excluded, but it's one of the "seed" positions so it's
already been decoded), but making the completely wrong sanity check and
sending us into oob, undefined behavior territory.

src/lib/cimb_translator/FloodDecodePositions.cpp

@@ -103,10 +103,10 @@ int FloodDecodePositions::update(unsigned index, const CellDrift& drift, unsigne
  {
  std::array<int,4> horizon = {-1, -1, -1, -1};
  horizon[0] = _cellFinder.right(rridx);
- if (horizon[0])
+ if (horizon[0] >= 0)
  horizon[1] = _cellFinder.right(horizon[0]);
  horizon[2] = _cellFinder.left(llidx);
- if (horizon[2])
+ if (horizon[2] >= 0)
  horizon[3] = _cellFinder.left(horizon[2]);
 
  update_adjacents(horizon, drift, error_distance, cooldown);
@@ -118,10 +118,10 @@ int FloodDecodePositions::update(unsigned index, const CellDrift& drift, unsigne
  {
  std::array<int,4> vert = {-1, -1, -1, -1};
  vert[0] = _cellFinder.top(uuidx);
- if (vert[0])
+ if (vert[0] >= 0)
  vert[1] = _cellFinder.top(vert[0]);
  vert[2] = _cellFinder.bottom(ddidx);
- if (vert[2])
+ if (vert[2] >= 0)
  vert[3] = _cellFinder.bottom(vert[2]);
 
  update_adjacents(vert, drift, error_distance, cooldown);