win: fix returning thread id in uv_thread_self
win: fix returning thread id in uv_thread_self

The main thread does not have the thread id TLS created.

Fixes: https://github.com/joyent/node/issues/25602
PR-URL: https://github.com/libuv/libuv/pull/418
Reviewed-By: Alexis Campailla <37ac5e111a9b2f779e373f78efa4f7678b93feb1@janeasystems.com>
Reviewed-By: Bert Belder <9f2ff180696da386c1be31b4507155616db25dc6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso