[ELF.h] - Check the case when e_shstrndx=SHN_XINDEX, but the corresponding sh_link is broken.

When e_shstrndx is equal to SHN_XINDEX,
the index of the section string table section should
be taken from the sh_link field of the section
header at index 0.

If sh_link is broken, e.g. contains an index that is
larger than number of sections, then error is reported.

This error message was untested before.

Differential revision: https://reviews.llvm.org/D65391

llvm-svn: 368139