ld: Check for ELF input before accessing ELF section data

ld: Check for ELF input before accessing ELF section data

commit b209b5a6b8a accesses ELF section data without checking if input is
ELF.  It caused:

sh: line 1: 1355479 Segmentation fault      (core dumped) /export/build/gnu/tools-build/binutils-gitlab-x32/build-x86_64-linux-gnux32/ld/ld-new -o tmpdir/pe-x86-64-1 -z norelro -L/export/gnu/import/git/gitlab/x86-binutils/ld/testsuite/ld-x86-64 -m elf_x86_64 --entry=begin tmpdir/pe-x86-64-1a.obj tmpdir/pe-x86-64-1b.obj tmpdir/pe-x86-64-1c.obj 2>&1
FAIL: Build pe-x86-64-1

on Linux/x86-64 with PE/x86-64 inputs.  Add check for ELF input before
accessing ELF section data.

	* ldelfgen.c (add_link_order_input_section): Check for ELF input
	before accessing ELF section data.