Make sure to call `curlx_now_init()` before the first call to
`curlx_now()`.
Before this patch the first `curlx_now()` used the non-Vista code path
calling `GetTickCount()` on Vista+. This is harmless, but the upcoming
PR #18009 is going to drop the non-Vista code path, causing a division
by zero at startup in test servers, without this fix.
Bug: https://github.com/curl/curl/pull/18009#issuecomment-3652154307Closes#19973
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