[Object/ELF] - Do not crash on invalid Header->e_shoff value.

sections_begin() may return unalignment pointer when Header->e_shoff isinvalid.
That may result in a crash in clients, for example we have one in LLD:

assert((PtrWord & ~PointerBitMask) == 0 &&
       "Pointer is not sufficiently aligned");
fails when trying to push_back Elf_Shdr* (unaligned) into TinyPtrVector.

Patch forces check for alignment of Header->e_shoff.

Differential revision: https://reviews.llvm.org/D25368

llvm-svn: 283740