Don't die in try_unsafe_borrow if tls isn't ready

If there's no TLS key just yet, then there's nothing to unsafely borrow, so
continue returning None. This prevents causing the runtime to abort itself when
logging before the runtime is fully initialized.

Closes #9487
auto merge of #9490 : alexcrichton/rust/issue-9487, r=cmr

If there's no TLS key just yet, then there's nothing to unsafely borrow, so
continue returning None. This prevents causing the runtime to abort itself when
logging before the runtime is fully initialized.

Closes #9487

r? @brson