atomic: Fix type error with clang++
clang++ checks the 2nd args of __atomic_compare_exchange_n() has the same type as the first, which fails when 2nd arg is nullptr which is of type nullptr_t.
Fixes issue #2226 (closed).
clang++ checks the 2nd args of __atomic_compare_exchange_n() has the same type as the first, which fails when 2nd arg is nullptr which is of type nullptr_t.
Fixes issue #2226 (closed).