elfcopy: fail if debug link target is empty

An empty debug link target previously returned a confusing and incorrect
error like "objcopy: fread failed: No error: 0". Now, return an explicit
error.

GNU objcopy allows an empty file as the debug link target. However,
that case is nonsensical so diverging from GNU behaviour is fine.

Reviewed by: bdrewery
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5541