[obj2yaml] - Fix the crash in getUniquedSectionName().

`getUniquedSectionName(const Elf_Shdr *Sec)` assumes that
`Sec` is not `nullptr`.

I've found one place in `getUniquedSymbolName` where it is
not true (because of that we crash when trying to dump
unnamed null section symbols).

Patch fixes the crash and changes the signature of the
`getUniquedSectionName` section to accept a reference.

Differential revision: https://reviews.llvm.org/D93754