crypto: store thread id as pointer-sized

In https://github.com/MSOpenTech/libuv/commit/59658a8de7cc05a58327a164fd2ed4b050f8b4f4
the return of uv_thread_self() was changed from unsigned long to
uv_thread_t.

uv_thread_t is a HANDLE (pointer-sized) on Windows, which means that
on Win64 it cannot be stored with CRYPTO_THREADID_set_numeric without
data loss.

Furthermore, without this change there will be a build break on Windows
when the libuv change is integrated into Node, because of the
conversion from HANDLE to unsigned long.

Other related commits:
https://github.com/joyent/node/commit/5845a6bcd5b36168bdddeb85da8d8d9d36de7642
https://github.com/MSOpenTech/libuv/commit/919d8ec63ac53566ad1f090058ec15966bd0e960
I spent a bunch of time trying your suggestion and some other variants, only to convince myself that the old-style cast combined with the assert is the most reliable solution. It seems that MSVC++ 2010 does't completely adhere to the standard regarding reinterpret_cast behavior.