objdump: Handle elf_getarhdr returning NULL in handle_ar

objdump: Handle elf_getarhdr returning NULL in handle_ar

Report of the static analyzer:
Pointer, returned from function 'elf_getarhdr' at objdump.c:314,
may be NULL and is dereferenced at objdump.c:317. (CWE-476, CWE-690)

Corrections explained:
When processing archive elements, the code could dereference a NULL
pointer if 'elf_getarhdr' returns NULL. This patch adds a check to
ensure 'arhdr' is not NULL before using it. The fix ensures that the
function safely handles cases where 'elf_getarhdr' fails, avoiding
potential crashes.

Triggers found by static analyzer Svace.

Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>