Recommit rr366796 "[Object/ELF.h] - Improve testing of the fields in ELFFile<ELFT>::sections()."

With a fix of the issue found by UBSan.

Original commit message:

This eliminates a one error untested and
also introduces a error for one more possible case
which lead to crash previously.

Differential revision: https://reviews.llvm.org/D64987

llvm-svn: 366886