Rollup merge of #148026 - joboet:dont-leak-thread-closure, r=Mark-Simulacrum

std: don't leak the thread closure if destroying the thread attributes fails

The comment about double-free is wrong – we can safely drop both the thread attributes and the thread closure. Here, I've used `DropGuard` for the attributes and moved the `Box::into_raw` to just before the `pthread_create`.

Author: matthiaskrgr