ar: Check elf_getahdr doesn't return NULL

ar: Check elf_getahdr doesn't return NULL

When elf_getahdr returns NULL we shouldn't even try to handle the ar
header, but immediately go to the next entry.

	* src/ar.c (do_oper_extract): If elf_getahdr goto next.
	(do_oper_delete): Likewise.
	(do_oper_insert): Likewise.

Suggested-by: Anton Moryakov <ant.v.moryakov@gmail.com>
Signed-off-by: Mark Wielaard <mark@klomp.org>