elf_getaroff: Fix elf_getaroff error return value

elf_getaroff: Fix elf_getaroff error return value

elf_getaroff currently returns ELF_C_NULL (0) to indicate that an error
occured (ex. the Elf descriptor is not associated with an archive).

However elf_getaroff is intended to return -1 if an error occurs.
eu-ar assumes -1 indicates an error and other libelf implementations
use -1 to indicate an error in elf_getaroff.

Replace ELF_C_NULL with -1 as elf_getaroff's error return value.

Signed-off-by: Aaron Merey <amerey@redhat.com>