[ELF] Check if LinkSec is nullptr when initializing SHF_LINK_ORDER sections

Summary: This protects lld from a null pointer dereference when a faulty input file has such invalid sh_link fields.

Reviewers: ruiu, espindola

Reviewed By: ruiu

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D51743

llvm-svn: 341611