util: Don't reference errno when pthread fails.

Summary:
Pthread library does not set errno.
Pthread library's errno is returned by return value.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

This is a backport of core#19194

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9524