Code refactoring of GC_destroy_thread_local (move assertion on 'tlfs'

Code refactoring of GC_destroy_thread_local (move assertion on 'tlfs'
out of function since otherwise assertion is skipped if HANDLE_FORK)

* pthread_support.c (GC_unregister_my_thread_inner): Add assertion on
"tlfs" value before GC_destroy_thread_local() invocation.
* win32_threads.c (GC_unregister_my_thread, GC_thread_exit_proc):
Likewise.
* thread_local_alloc.c (GC_destroy_thread_local): Remove assertion on
"tlfs" value (since it was skipped if HANDLE_FORK because of the
function invocation from GC_remove_all_threads_but_me).