There's a funny circular dependency in glibc sources because errno lives in the TLS block which is allocated using __sbrk which can set the errno before it's allocated (see the __libc_setup_tls).

The branch that actually touches the errno is unlikely to be executed. However I did experience a puzzling crash with a cross-compiled libc because the compiler was smart enough to inject a speculative load of errno outside of the branch. Fun times.