gh-128679: Fix tracemalloc.stop() race conditions (#128893)

tracemalloc_alloc(), tracemalloc_realloc(), tracemalloc_free(),
_PyTraceMalloc_TraceRef() and _PyTraceMalloc_GetMemory() now check
'tracemalloc_config.tracing' after calling TABLES_LOCK().

_PyTraceMalloc_TraceRef() now always returns 0.