Fix buffer overflow when registering a dedicated build server on MS

When registering a server that runs on the dedicated server build on Unix, the server could potentially crash due to a buffer overflow when allocating a thread condition, as the allocation was incorrectly configured to allocate a thread_t instead of a thread_cond_t. The bug itself occurs very inconsistently since it relies on a race condition to trigger, so reproducing the bug might not be the easiest to do.