Fix wrap-aware isAfter function in Congestion.cxx

Fix wrap-aware isAfter function in Congestion.cxx

Result of overflow on signed integer arithmetic is undefined in C/C++ standard.
So in previous version clang was compiling the statement as (int)a > (int)b (i.e. assuming no overflow), which leads to incorrect result.

Correct deterministic behavior means doing overflow arithmetic as unsigned, i.e.
 a != b && a - b <= UINT_MAX / 2