[llvm-readobj] - Do not crash when GnuHashTable->symndx is greater than the dynamic symbols count.

`Elf_GnuHash_Impl` has the following method:

```
ArrayRef<Elf_Word> values(unsigned DynamicSymCount) const {
  return ArrayRef<Elf_Word>(buckets().end(), DynamicSymCount - symndx);
}
```

When DynamicSymCount is less than symndx we return an array with the huge broken size.
This patch fixes the issue and adds an assert. This assert helped to fix an issue
in one of the test cases.

Differential revision: https://reviews.llvm.org/D81937