[llvm-readobj] - Fix possible crashes related to dumping gnu hash symbols.

It fixes possible scenarios when we crash/assert with `--hash-symbols` when
dumping an invalid GNU hash table which has a broken value in the buckets array.

This fixes a crash reported in comments for
https://bugs.llvm.org/show_bug.cgi?id=47681

Differential revision: https://reviews.llvm.org/D88561