[ELF] -r: fix crash when SHF_LINK_ORDER linked-to section has a larger index

Fixes: b8dface2

ThinLTO asan build may place `asan_globals` before the associated `.bss.xxx` section.
`rel->getOutputSection()` is nullptr because `rel->parent` hasn't been
set, leading to a crash. Simplify return `s->name` in this case.