Fix failing assert when disappear-at-goal is set

The unique cell computation was incorrect, as the first component of
State is time, not a coordinate.

Fixes issue #28

example/cbs.cpp

@@ -430,7 +430,7 @@ class Environment {
       // This is a trick to avoid changing the rest of the code significantly
       // After an agent disappeared, put it at a unique but invalid position
       // This will cause all calls to equalExceptTime(.) to return false.
-      return State(-1 * agentIdx, -1, -1);
+      return State(-1, -1 * (agentIdx + 1), -1);
     }
     return solution[agentIdx].states.back().first;
   }

example/ecbs.cpp

@@ -498,7 +498,7 @@ class Environment {
       // This is a trick to avoid changing the rest of the code significantly
       // After an agent disappeared, put it at a unique but invalid position
       // This will cause all calls to equalExceptTime(.) to return false.
-      return State(-1 * agentIdx, -1, -1);
+      return State(-1, -1 * (agentIdx+1), -1);
     }
     return solution[agentIdx].states.back().first;
   }